Final answer:
A hand and arm massage during a microdermabrasion treatment is best performed while the mask is on, as it is a time when the client is resting and the mask is working.
Step-by-step explanation:
The optimal time to perform a hand and arm massage during a microdermabrasion treatment would be c) while the mask is on. During this step, the client is typically in a resting state, and the mask requires time to exert its effect on the skin, which provides an excellent opportunity to offer a relaxing massage. Not only does this enhance the overall spa experience, but it also helps to keep the client relaxed during a period that would otherwise be inactive.