Final answer:
Systematic desensitization is a therapy used to treat phobias by gradually exposing the person to the feared object or situation while practicing relaxation techniques.
Step-by-step explanation:
Systematic desensitization is a form of therapy used to treat phobias or extreme fears. It involves gradually exposing the client to the feared object or situation while practicing relaxation techniques. In the case of a client with an extreme fear of elevators, option C would be appropriate. This option suggests gradually exposing the client to an elevator while simultaneously practicing relaxation techniques.