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

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

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

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

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

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.