What is the maximum number of ESX/ESXi Hosts that can be managed with vCenter Foundation?

A.
5

B.
6

C.
3

D.
10

Explanation:
VMware vSphere 4 Pricing, Packaging and Licensing Overview Whitepaper, page 8.

VMware vCenter Server Foundation. Provides powerful management tools for smaller environments (up to three VMware vSphere hosts) looking to rapidly provision, monitor and control virtual machines.