Which rocket did they launch?

Which word is the direct object?

Answer: Rocket is a direct object.

Explanation:

A direct object of a sentence is a noun/pronoun that receives the action performed by the subject. One way to identify a direct object is to answer the questions - 'what?' or 'who?'

In this sentence, the word 'rocket' is a direct object, as it receives the action of launching. It can also be identified if we ask the following question: ''What did they launch?''. The answer is (A particular kind of) a rocket.