Final answer:
When a client is asked to purse their lips, it involves the contraction of the orbicularis oris muscle which regulates the opening and closing of the mouth.
Step-by-step explanation:
The action described in the question, asking the client to purse their lips, involves the movement of the muscles around the face, specifically the orbicularis oris muscle.
The orbicularis oris muscle is responsible for regulating what comes in and goes out of the mouth. When the client purses their lips, the orbicularis oris muscle contracts, causing the lips to form a narrow opening.