Choose the correct alternative which will improve the part of the sentence given in quotes.
It is raining, "isn't it"?

Correct option is D

No improvement is needed as "isn't it" is correctly used.
Explanation: "Isn't it" is the correct tag question form following a positive statement in present continuous tense, "It is raining." The tag is used to confirm or seek confirmation, and it correctly mirrors the auxiliary verb and subject of the main clause.
Grammatical rule used: Tag questions are formed using the auxiliary verb from the main clause and changing the polarity; if the statement is positive, the tag is negative, and vice versa.
Example: She's coming soon, isn't she?