The word in the sentence that is the direct object is Rocket.
Explanation:
The direct object is a noun or noun group that is directly affected by the action of the subject, it can be easily identified because it answers the questions of who(m) or what, in the sentence Which rocket did they launch? The subject is they and the action of the subject is launch then the rocket is the one who receives the action.
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, <em>rocket</em>is the direct object because it receives the action <em>launch</em>. Furthermore, we can also identify it as the direct object when we ask the question "What did they launch?" Answer: (Some kind of) rocket.
