The security investigation tool is specifically designed for investigating security incidents like spam and phishing emails. It allows you to search for emails, review their details, and determine the scope of the incident, including identifying whether other users were affected. This tool is the most appropriate and efficient way to respond to the incident.

E Implement two-factor authentication for all users
Explanation:
Conduct regular security awareness training to educate users: Educating users about phishing threats and safe online practices can help them recognize and avoid phishing attempts, reducing the chances of them falling for such scams.
Create a Drive trust rule that blocks all external domains except for a pre-approved list of trusted partners: By setting up a Drive trust rule to limit access to files from external domains, you can block links to malicious files hosted on untrusted external Google Drives while still allowing access to legitimate external files from trusted sources.

Since user accounts are managed in Active Directory (AD) and synced to Google Workspace via Google Cloud Directory Sync (GCDS), the best approach to enforce the new password policy is to create the password policy within Active Directory and then enable password synchronization in GCDS. This ensures that the complex password requirements are enforced within AD, and when passwords are updated, they will be synchronized with Google Workspace, maintaining consistency across both systems.

Switching the former employee's account to an Archived User license ensures that their data and documents are preserved, and access is retained for the manager and team without incurring the full cost of an active Workspace license. Archived User licenses are a cost-effective way to maintain access to documents while preventing unauthorized access to the account.

A data loss prevention (DLP) rule with the predefined credit card number detector will help you identify and prevent the accidental sharing of files that contain sensitive credit card information. Setting the action to "block external sharing" ensures that such files cannot be shared externally. Enabling the "Log event" option will record any incidents of external sharing for auditing and reporting purposes, fulfilling both the security and reporting requirements.