Which scaling type should you use?

You are deploying an application to App Engine. You want the number of instances to scale based on request rate. You need at least 3 unoccupied instances at all times.

Which scaling type should you use?
A . Manual Scaling with 3 instances.
B . Basic Scaling with min_instances set to 3.
C . Basic Scaling with max_instances set to 3.
D . Automatic Scaling with min_idle_instances set to 3.

Answer: D

Explanation:

Reference: https://cloud.google.com/appengine/docs/standard/python/how-instances-are-managed