When there's a legal investigation, the priority is to ensure that relevant data is preserved and not deleted, regardless of retention policies or user actions. A "hold" (also known as a litigation hold or legal hold) in Google Vault is specifically designed for this purpose. It overrides all retention rules (both default and custom) and prevents any data covered by the hold from being purged, even if a user attempts to delete it.

Here's why the other options are not the correct or best solution:

A. Change the Vault default retention rule to one year instead of five. Changing the default retention rule would affect all Drive data in your organization, not just this specific employee's. It's a broad change and not suitable for a targeted legal hold. Moreover, it wouldn't guarantee preservation against user deletions.

B. Place the employee into a separate organizational unit (OU). Create a custom one-year retention rule for this OU. While creating custom retention rules for OUs is possible, it's not the primary mechanism for a legal hold. Retention rules define when data can be deleted, but a hold prevents deletion irrespective of the retention period. If the employee deletes the data, a retention rule won't stop it from moving to trash (and eventually being purged) unless a hold is in place. Furthermore, a one-year retention rule isn't the goal; the goal is to preserve for "at least one year" (meaning indefinitely until the hold is released). The default five-year rule is already longer than one year, but doesn't override user deletion.

D. Confirm that the Vault default retention rule is set for five years. The question states that the default retention rule for Drive is already set for five years. While this is good for general data retention, it does not prevent a user from deleting their own files from Drive, nor does it specifically address the need for a legal hold where data must be absolutely preserved. A default retention rule does not override user deletion or ensure data preservation for legal purposes.

References from Google Workspace Administrator:

Holds in Google Vault: This is the core concept. Holds prevent data from being purged from Google systems, regardless of retention rules or user actions, until the hold is released. They are specifically used for legal discovery or investigation purposes.