Answer:
C is a sentence fragment.
Explanation:
Sentence fragments are groups of words that look like sentences, but aren't. In order to be a sentence, groups of words need at least to have one independent clause. An independent clause is any group of words that contain both a subject and a verb and can stand alone as a sentence since it represents a complete thought.
The sentence above is not an independent clause, but a dependent one. It does not represent a complete thought, and therefore fails to stand on its own (see 1). The clause "he had built as a kid" modifies the noun "tree fort", so the pronoun "he" acts as the subject of that IP but it is not the subject of the clause. The subject of "hiding in the tree fort he had built as a kid" is an empty category linked to the overt subject of the main clause, yet there is no main clause, so the sentence lacks both the subject and the verb functioning as the predicate.
1) *Hiding in the tree fort he had built as a kid
