The correct answer is C. The girl in the photo is my cousin, I think you have met her before.
Explanation:
In grammar, independent clauses can only be separated using a period, a semicolon or a comma followed by a conjunction, because of this using only a comma to separate to independent clauses is a grammatical mistake known as a comma splice. In option C "The girl in the photo is my cousin, I think you have met her before" there is comma splice because in this sentence there are two independent clauses or complete sentences "The girl in the photo is my cousin" and "I think you have met her before" and these have been separated only using a comma which occurs in comma splice.
