To enforce different external sharing policies for different departments within the same Google Workspace domain, you should use Google Drive sharing policies configured at the organizational unit (OU) level. Drive trust rules are the mechanism within Google Workspace to control how users can share files inside and outside the organization.  

Here's why option A is correct and why the others are not the most appropriate solutions:

A. Create a Drive trust rule that allows external sharing for the Research and Development organizational unit (OU) and another rule that blocks external sharing for the Finance OU.

Google Workspace allows administrators to set specific Drive sharing settings for different organizational units. By creating a Drive trust rule (or more accurately, configuring the external sharing options within Drive and Docs settings for each OU), you can enable external sharing for the Research and Development OU while simultaneously restricting or completely blocking external sharing for the Finance OU. This granular control at the OU level directly addresses the requirement of having different policies for the two departments.  

Associate Google Workspace Administrator topics guides or documents reference: The official Google Workspace Admin Help documentation on "Control how users can share Drive files externally" (or similar titles) explains how to manage external sharing options at the organizational unit level. This includes:Setting sharing options by organizational unit: The documentation details how to navigate to Apps > Google Workspace > Drive and Docs > Sharing settings in the Admin console and then select a specific organizational unit to customize its sharing permissions.  

Controlling sharing outside your organization: This section explains the various settings available, including allowing sharing with anyone, only with specific domains, or completely preventing external sharing.

While the term "Drive trust rule" might be used in more advanced contexts related to trusted domains, the core functionality of controlling external sharing based on OUs is the key here. The settings within the Drive and Docs sharing configuration for each OU achieve the desired outcome.

B. Enable Vault for the Finance organizational unit (OU) to ensure that all files shared externally are retained and auditable.

Google Vault is used for eDiscovery, legal holds, and retention of data. While it can retain and audit externally shared files (if sharing is allowed), it does not prevent external sharing. Enabling Vault for the Finance OU would not block them from sharing files externally; it would only ensure that if they do, those shared files are preserved and can be audited. This does not meet the requirement of blocking external sharing for the Finance department.  

Associate Google Workspace Administrator topics guides or documents reference: The Google Workspace Admin Help documentation on Google Vault clearly outlines its purpose and functionalities, which are focused on data retention, legal holds, and search/export for compliance and legal reasons, not on preventing sharing.  

C. Apply an organization-wide data loss prevention (DLP) rule that scans for sensitive information and prevents external sharing of those files. Apply that rule to the Finance organizational unit (OU).  

While DLP rules can prevent the external sharing of files containing sensitive information, they are triggered by the content of the files, not by a blanket restriction on all external sharing for a specific OU. The requirement is to block all external sharing for the Finance department, regardless of the content. Applying a DLP rule only to the Finance OU might be complex to manage for a complete block and is not the most direct way to achieve the stated goal. OU-based sharing settings are more straightforward for this purpose.  

Associate Google Workspace Administrator topics guides or documents reference: The Google Workspace Admin Help documentation on Data Loss Prevention (DLP) explains how to create rules based on content to prevent sensitive data leaks. While DLP can control sharing, it's not the primary mechanism for completely blocking all external sharing for an entire OU.  

D. Create a separate Google Workspace domain for the Finance organizational unit (OU) and disable external sharing for that domain.

Creating a separate Google Workspace domain for the Finance department is an overly complex and administratively burdensome solution. It would involve managing two separate domains, user accounts, billing, and potentially complicate internal collaboration between departments. Using organizational units within the same domain provides a much more efficient and manageable way to apply different policies.

Associate Google Workspace Administrator topics guides or documents reference: Google Workspace's organizational unit structure is specifically designed to allow administrators to apply different settings and policies to groups of users within a single domain, avoiding the need for separate domains for policy enforcement.  

Therefore, the most direct and appropriate solution is to configure the Google Drive sharing settings at the organizational unit level, allowing external sharing for the Research and Development OU and blocking it for the Finance OU.