Which cranial nerve should be assessed to evaluate a person's ability to frown?

The assessment of a person's ability to frown specifically involves the facial nerve, also known as cranial nerve VII. This nerve is responsible for controlling the muscles of facial expression, including those that allow for frowning. When you ask someone to frown, you are testing the function of the facial nerve, which innervates the muscles responsible for that particular movement.

In contrast, the olfactory nerve (cranial nerve I) is primarily involved in the sense of smell and does not relate to facial expressions. The trigeminal nerve (cranial nerve V) is associated with sensation in the face and motor functions such as chewing, but it does not directly control the muscles used for facial expressions like frowning. The vagus nerve (cranial nerve X) has a variety of roles, particularly in autonomic control of the heart and digestive tract, and is not involved in facial muscle control. Thus, the facial nerve is the relevant nerve to evaluate when assessing the ability to frown.