The correct answer is D because using composite objects and parallel uploads can significantly speed up the upload process to Cloud Storage. Signed URLs provide a secure, time-limited access to the file without requiring the contractors to have a Google account. This method is both efficient and secure. Option A is incorrect because Bigtable is not designed for file storage. Option B is incorrect as it grants access to all authenticated Google account holders, which is not secure. Option C is overly complex and not the quickest solution for sharing files.