D is the answer.
https://cloud.google.com/build/docs/subscribe-build-notifications
Cloud Build publishes messages on a Google Pub/Sub topic when your build's state changes, such as when your build is created, when your build transitions to a working state, and when your build completes.
The Pub/Sub topic to which Cloud Build publishes these build update messages is called cloud-builds. Each message contains a base64 JSON string representation of your Build resource in the message.data attribute. The build's unique ID and the build's status can be found in the message.attributes field.
You can use a push or pull model for your Pub/Sub subscriptions.
https://cloud.google.com/build/docs/subscribe-build-notifications#push
Push subscriptions deliver messages to an HTTP endpoint that you define. Messages are delivered as soon as they are published to the topic.