A is correct because this is the standard use case.
B is not correct because Cloud Bigtable is only a database.
C is not correct because Cloud Pub/Sub is only a messaging system.
D is not correct because Cloud Composer is not a good fit to receive input from users.

A/B testing requires distributing traffic in a controlled, randomized manner between different versions of your application. Weighted traffic splitting allows you to direct a specific percentage of traffic to the new version (with the new algorithm) and the rest to the current version. This approach provides a statistically valid way to compare the two versions' performance and assess the impact on sales.
HTTP header-based routing could target specific user segments but would not ensure random distribution for A/B testing. Mirroring traffic duplicates requests without user interactions being served by the new version, which doesn't provide direct test results. Enabling the feature on a single instance does not provide a randomized, scalable test approach.

This approach requires the least amount of effort and utilizes built-in integrations within Google Cloud. By configuring Artifact Registry to publish a message to a Pub/Sub topic each time a new image is pushed, you can directly trigger the vulnerability scan pipeline with this Pub/Sub message. This solution is efficient because it automates the process without requiring additional components like Cloud Functions or Cloud Scheduler, and it only triggers when new images are pushed, meeting your requirement for scanning upon each image push.

https://cloud.google.com/monitoring/api/metrics_agent#agent-memory
By default Memory metrics is not collected). To double confirm. Just goto Console->Operations->Monitoring->Dashboards->VM Instances->Memory Tab (Assume you have VM running already). You will see a info message saying that No agents detected.
Monitoring agents collect memory metrics, disk metrics, and more. Learn more about agents and how to manage them across multiple VMs.

https://cloud.google.com/run/docs/container-contract#metadata-server
Cloud Run container instances expose a metadata server that you can use to retrieve details about your container instance, such as the project ID, region, instance ID or service accounts.
You can access this data from the metadata server using simple HTTP requests to the
http://metadata.google.internal/ endpoint with the Metadata-Flavor: Google header: no client libraries are required.