The correct answer is A because using Cloud Storage signed URLs allows for quick and secure sharing of files with third parties without requiring them to have a Google account. Signed URLs provide time-limited access to the resource, which automatically expires after the specified duration (4 hours in this case). This method is efficient and adheres to Google's best practices for secure file sharing. Options B and D involve unnecessary manual effort and are not quick solutions. Option C is incorrect as granting allUsers access is not secure and would make the files public.