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? 

Answer options:

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.

D

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