Final answer:
The correct relative pronoun to complete the questioned sentence is 'that'. It is used to introduce information critical to identifying the sleeping bag in question. The other options, 'who', 'whom', and 'which', do not fit in this context.
Step-by-step explanation:
The question "Is this the sleeping bag you bought for the camping trip?" is looking for a relative pronoun that can introduce a defining clause, which gives essential information about the sleeping bag. The correct option here is c. that because it refers to the sleeping bag being the direct object of the verb 'bought'. The other options, 'who' and 'whom', are used for people, not things, and 'which' is often used for non-defining (additional) clauses. So the complete sentence would be: "Is this the sleeping bag that you bought for the camping trip?"
Just as a clarification, it's important to note the distinction between 'who's' and 'whose'. 'Who's' is a contraction for 'who is' or 'who has', as in the sentence 'Who's going to the game?'. On the other hand, 'whose' is the possessive form of 'who', used to inquire about ownership, as in 'Whose backpack is this?'.