The correct answer is (C) To make unaware of, or indifferent in feeling, or the like. To lessen an abnormal sensitivity of one's body to an external stimulus, such as a particular allergen. In photography, to render less sensitive or entirely insensitive to light, as an emulsion on a film. To render insensitive.

Examples of use:

• His wartime experience has desensitized him to many of life's minor irritations.
• The dentist uses a local anesthetic to desensitize the patient to pain in that area of the mouth where he intends to work.