Workforce Identity Federation is the modern, Google-recommended way to grant external partners access to Google Cloud resources using their own identity provider (IdP). This avoids the "Identity Lifecycle Management" burden of creating guest accounts in your own directory.

According to Google Cloud Documentation (Workforce Identity Federation Overview):

"Workforce Identity Federation lets you use an external identity provider (IdP) to authenticate and authorize a workforce—a group of users, such as employees, partners, and contractors—so that the users can access Google Cloud services. With Workforce Identity Federation, you don't need to synchronize user identities from your existing IdP to Google Cloud identities."

Advantages of this approach:

Syncless: You don't create or manage partner accounts in your Cloud Identity/Workspace (eliminating Option C).

Security: If a partner employee leaves their company, their access to your Google Cloud database is automatically revoked when their home IdP account is disabled.

Scoped Access: You grant IAM roles (like roles/cloudsql.client) specifically to the Workforce Pool or specific groups within that pool, ensuring they can't touch other resources.

Why other options are incorrect:

A is incorrect: Public IPs are a major security risk and don't provide centralized identity governance.

B is incorrect: You cannot "grant access" to accounts in another organization's Cloud Identity directly in a secure, manageable way for production databases without federation.