You have developed an application using Cloud ML Engine that recognizes famous paintings from uploaded images. You want to test the application and allow specific people to upload images for the next 24 hours. Not all users have a Google Account. How should you have users upload images?

The correct answer is to use a signed URL. Signed URLs grant temporary, time-limited access to a specific Cloud Storage object to anyone who possesses the URL, without requiring a Google Account. This directly addresses the requirement for non-Google Account users and the 24-hour time limit. Uploading to Cloud Storage with a password that expires is not a standard or secure method for Cloud Storage access. Cloud Storage buckets are secured via IAM policies, not passwords. Creating an App Engine application is an unnecessary overhead for a temporary image upload task. While it could handle uploads, it introduces complexity and cost that a signed URL avoids. Furthermore, authenticating via Cloud Identity would require users to have Google Accounts, which contradicts the problem statement.