Option B is correct
An app profile specifies the routing policy that Bigtable should use for each request.
Single-cluster routing routes all requests to 1 cluster in your instance. If that cluster becomes unavailable, you must manually fail over to another cluster.
Multi-cluster routing automatically routes requests to the nearest cluster in an instance. If the cluster becomes unavailable, traffic automatically fails over to the nearest cluster that is available. Bigtable considers clusters in a single region to be equidistant, even though they are in different zones. You can configure an app profile to route to any cluster in an instance, or you can specify a cluster group that tells the app profile to route to only some of the clusters in the instance.
Cluster group routing sends requests to the nearest available cluster within a cluster group that you specify in the app profile settings.
Reference:-https://cloud.google.com/bigtable/docs/app-profiles#routing