D is correct because Cloud Run is a managed service that requires minimal administration. It allows for automatic scaling according to load, making it an ideal choice for deploying microservices. Cloud Run abstracts away infrastructure management, automatically scaling up or down based on traffic. This option provides the best balance between scalability and minimal administrative overhead.

A is incorrect because managed instance groups require more administration than Cloud Run. This approach would necessitate manual scaling and administration, lacking the automatic scalability feature of Cloud Run.

B is incorrect as, although Google Kubernetes Engine (GKE) offers scalability, it demands more management than Cloud Run. GKE requires manual scaling and configuration, making it less suitable for scenarios aiming for minimal administration.

C is incorrect because combining the services into a single Docker image and deploying on App Engine may not offer optimal scalability. This approach could lead to a monolithic architecture, contrary to microservices principles, and might not scale as efficiently as Cloud Run.

For more details, visit: Cloud Run Quickstarts