Choose a managed compute product for an MVP used only during business hours that automatically scales to zero to avoid idle costs.

Cloud Functions is the correct choice because it is a serverless, event-driven compute platform that automatically scales to zero when not in use, perfectly aligning with the requirement to avoid idle costs for an MVP used only during business hours. Compute Engine provides virtual machines that incur costs even when idle unless manually shut down, making it unsuitable for automatic cost avoidance. Google Kubernetes Engine (GKE) is a container orchestration platform that requires managing clusters, which incur costs even when no applications are running, and doesn't scale to zero at the cluster level. App Engine flexible environment also runs continuously and does not automatically scale to zero, leading to idle costs.