Interchange interrogative and assertive sentences:
John is a sincere boy.
A)Isn't John a sincere boy?
B)Is John a sincere boy?
C)No change.
D)A & B both.

Hint: Interrogative sentences are the ones that ask questions and end with a question mark. Assertive sentences are the sentences that give us information.

Complete step-by-step answer:

We have been given an assertive sentence and we have to interchange it with the interrogative sentences. From the hint given to us, we know that interrogative sentences are the ones that ask questions and end with a question mark. We also know that assertive sentences are the sentences that give us information.
To convert any sentence into an interrogative sentence, the verb should be placed first, followed by the subject and then the object. The verb in" the sentence that has been given to us is, ‘is’.
Thus, we can write the interrogative sentence as, “Is John a sincere boy?”
The verb can be followed by a negative word in order to make the interrogative sentence negative.
Thus, we can write the second interrogative sentence as, “Isn’t John a sincere boy?”
When the verb is followed by a negative word to form a negative interrogative sentence, a positive response is expected. Similarly, when the interrogative sentence is positive, a negative response is expected.
Thus, option D is the correct answer as both options A and B are correct.

Note: An assertive sentence is more popularly known as a declarative sentence. It asserts, declares, or informs us something.