Answer:
C. Hiding in the tree fort he had built as a kid.
Explanation:
Sentence fragments are a group of words that seem a sentence because they begin with a capital letter and end with a punctuation mark like a period, but that in reality, are not sentences because they do not express a complete thought.
From all the given choices, sentence D is the one that is a fragment because it does not express a complete thought: it is missing an independent clause that concludes the idea. Here's an example of how we can correct this sentence fragment:
Hiding in the tree fort he had built as a kid, he realized how tiny the fort was.
