The correct option is A isn't it
The verb in the first part of the sentence is "is". It's is the contracted form of " it is". Hence, the tag question will also be formed by the negative form of the same verb. Thus, the tag question will be " is not", or its contracted form, "isn't". Therefore, option A is the correct answer. Options B , C and D are all incorrect because they all constitute the incorrect verb in their respective question tags which is not compatible with the one in the first part of the question.