What is the meaning of a red, or critical, indicator on a DRS cluster?

A.
Red clusters are the highest priority and will receive resource allocations first

B.
At least one host in the cluster is attempting to go into Maintenance Mode

C.
Yellow clusters will become red after 15 minutes if DRS is unable to dynamically balance the load

D.
Red clusters indicate virtual machines are using more resources than the fixed reservation allows

Explanation:
Red DRS Cluster

A cluster enabled for DRS becomes red when the tree is no longer internally consistent and does not have enough resources available. For example, a DRS cluster turns red if the virtual machines in a fixed resource pool use more resources than the Reservation of that resource pool allows.