Answer:
D. Rocket
Step-by-step explanation:
A direct object is a noun or a pronoun that receives the action in the sentence; they can be identified by answering the “what?” or the “who?” questions.
In the given question, rocket is the direct object because it receives the action launch. Furthermore, we can also identify it as the direct object when we ask the question "What did they launch?" Answer: (Some kind of) rocket.