Option D is correct because a custom image is specific to your project and serves as a template for creating new VM instances. By creating a custom Compute Engine image from a snapshot, you can replicate the configuration and data of the base VM, enabling the launch of new instances to accommodate the increased traffic. This method is more efficient for scaling than using snapshots directly, which are primarily for backup and recovery. Images facilitate the creation of instances with identical configurations, whereas snapshots are used for disk backups or restorations. For more details, refer to Google Cloud's documentation on creating instances.