Final answer:
The answer to the question is 'C. sensitive,' as sensitive is considered a skin condition rather than a primary skin type like oily, dry, or combination.
Step-by-step explanation:
The question asks about skin types, which is a subject related to health and dermatology. The four commonly recognized skin types are oily, dry, combination, and normal. These skin types are distinguished by factors such as oil production, hydration levels, and sensitivity to skincare products or environmental factors.
Sensitive is often considered a skin condition or a skin reaction rather than a basic skin type. Conditions like allergies, rosacea, or contact dermatitis can cause skin sensitivity, but do not define the skin's baseline type. Therefore, the answer to the question "What is not one of the four skin types?" would be C. sensitive.