What does this indicate?

An administrator is upgrading an ESXi 5.5 host to ESXi 6.x and gets the following error:
MEMORY_SIZE

What does this indicate?

A.
Insufficient memory on the ESXi host to complete the upgrade.

B.
Insufficient memory for Auto Deploy to complete the upgrade.

C.
Insufficient memory in vCenter Server to complete the upgrade.

D.
Insufficient memory for Update Manager to complete the upgrade.