An existing VMware VI3 infrastructure will be upgraded to vSphere. The infrastrucure contains 250 ESX and ESXi Hosts in multiple HA/DRS clusters. Which of the following tools does VMware recommend using to upgrade the ESX Hosts in this infrastructure to ESX 4.x?

Answer: vCenter Update Manager

Explanation:
VMware recommends using vCenter Update Manager when upgrading a large infrastructure or an infrastructure with clustered hosts. The vSphere Host Update Utility is typically used to upgrade one host at a time, making it ideal for smaller installations. The esxupdate tool is used to apply software updates to ESX hosts (not ESXi) and is more appropriate of patch or bulletin updates. The vihostupdate tool is used to apply software updates to ESX and ESXi hosts and is more appropriate of patch or bulletin updates.