Answer: D) occurred in Philadelphia in 1787.
Explanation:
The subject of a sentence is the thing, animal or person (noun or noun phrase) that is dealt in or described within the sentence, or that performs the main action.
D. is the only fragment that we can correct by adding a subject: The Constitutional Convention occurred in Philadelphia in 1787.
The other options would not be corrected with a subject only, for example, fragment A needs the verb "was" before "established", fragment B needs a predicate, and fragment C needs another clause that completes its meaning, for example: "The Legislative branch was created in order to allow the government to create laws."
