Exposure to uv light may cause a person to develop signs of early aging on skin.

The correct answer to the statement: Exposure to U.V light may cause a person to develop signs of early aging on skin, would be: On his or her skin.

Explanation:

The reason for this being the correct answer, instead of simply saying his, and her, is because the sentence is speaking about people without establishing particularities, and also, because it is talking about "a person", not more than one individual. Since gender cannot be ascertained here for the specific individual, in which case the answer would be either his, or her, as appropriate, and since the sentence itself clarifies that it is "a person", then the way to make the sentence complete is by adding both possessive pronouns "his or her" to signify that this problem can happen on both genders, on an individual level.