Choose the sentence that uses the correct question tag:

The correct answer is Option (3) → Nobody has fed the cat since morning, have they?

Question tag after “nobody”:

“Nobody” makes the sentence negative, so the tag is positive. In the tag, “nobody” becomes “they”. The helping verb is “has”. With “they”, it becomes “have”.

Why the Other Options are Incorrect?

Option 1: The chemist’s shop opens at nine, isn’t it? — Incorrect. The given sentence says “opens”. It has no helping verb, so the tag must use “does”, not “is”.
Corrected: The chemist’s shop opens at nine, doesn’t it?

Option 2: Rina wasn’t at the bus stop this morning, wasn’t she? — Incorrect. The sentence has “wasn’t”, so it is negative. A negative sentence takes a positive tag.
Corrected: Rina wasn’t at the bus stop this morning, was she?

Option 4: Mr Rao’s daughter teaches at the music school, doesn’t he? — Incorrect. The tag uses the pronoun for the subject. The subject is “daughter”, not “Mr Rao”. So the pronoun is “she”.
Corrected: Mr Rao’s daughter teaches at the music school, doesn’t she?