Final answer:
DPM is the technology that can power hosts on and off based on demand, optimizing power usage and providing energy efficiency in a VMware vSphere environment. Thus, correct answer is option C.
Step-by-step explanation:
The technology that can power-off and on physical hosts as they are needed is DPM, which stands for Distributed Power Management. DPM is a feature of VMware vSphere that optimizes power usage by turning off physical hosts during periods of low usage and turning them back on when the demand for computing resources increases. This ensures energy efficiency and cost savings without impacting performance. It is part of VMware's DRS (Distributed Resource Scheduler), which automatically balances computing workloads with available resources.