What is the meaning of a red 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:
vSphere Resource Management Guide ESX 4.0 ESXi 4.0 vCenter Server 4.0, page 59.
A cluster enabled for DRS becomes invalid (red) when the tree is no longer internally
consistent, that is, resource constraints are not observed.
The total amount of resources in the cluster does not affect whether the cluster is red. A
cluster can be red, even if enough resources exist at the root level, if there is an
inconsistency at a child level.
You can resolve a red DRS cluster problem either by powering off one or more virtual
machines, moving virtual machines to parts of the tree that have sufficient resources, or
editing the resource pool settings in the red part.