The principle of least privilege requires granting only the necessary permissions. To enable a Cloud Function in Project A to write to a bucket in Project B, the function must use a service account from Project A (its own project) with the minimal role roles/storage.objectCreator assigned on the bucket in Project B.

• Option A incorrectly creates the service account in Project B, which cannot be used directly by the Cloud Function in Project A (service accounts are project-specific).
• Option B correctly creates a service account in Project A, deploys the function with it, and grants the required role on Project B's bucket, adhering to least privilege.
• Option C uses Project A's default App Engine service account, which has broad permissions (Editor role by default), violating least privilege.
• Option D uses Project B's default App Engine service account, which is invalid since the function in Project A cannot use a service account from another project without explicit impersonation setup (not mentioned here).