It is our problem, not . . . . . . . . A. there’s B. there C. theirs D. their

The correct answer is: D. their

The sentence is “It is our problem, not theirs.” The word “theirs” is the possessive form of the pronoun “they.” It refers to the problem that belongs to the people who are being addressed. In this case, the problem belongs to the people who are not the speaker.

The other options are incorrect because they do not make sense in the context of the sentence.

• “There’s” is a contraction of the words “there is” or “there are.” It does not make sense to say “It is our problem, not there is.”
• “There” is an adverb that means “in that place.” It does not make sense to say “It is our problem, not in that place.”
• “A” is an indefinite article that is used to refer to a general thing or person. It does not make sense to say “It is our problem, not a.”

I hope this explanation is helpful!