Your microservices architecture needs reliable asynchronous messaging. Which GCP service provides at-least-once delivery semantics?

Correct answer: Cloud Pub/Sub

Option A is correct: Cloud Pub/Sub is Google Cloud's fully managed asynchronous messaging service that guarantees at-least-once delivery, meaning every published message is delivered to subscribers at least one time, making it the right choice for reliable decoupled microservice communication. Option B is incorrect because Cloud Functions is an event-driven compute service, not a standalone messaging broker with delivery guarantees. Option C is incorrect because Cloud Tasks manages the execution of discrete, explicitly created tasks and is more suited to job queuing with throttling rather than general pub/sub messaging patterns. Option D is incorrect because Firestore is a document database, not a messaging service, and does not provide message delivery semantics.