What are the storage containers called in vSphere?

You are explaining virtual machine storage to a colleague. She asks you where the virtual
machine files are stored. What are the storage containers called in vSphere?

A.
VM Port Groups

B.
Clusters

C.
Data Centers

D.
Datastores

Explanation:
http://www.yellow-bricks.com/2012/01/16/enabling-hot-add-by-default-ccgabvirtualworld/