Choose the sentence that uses the correct question tag:

The correct answer is Option (3) → Don’t forget to lock the gate at night, will you?

Question tag after “Don’t ...”:

A sentence that begins with “Don’t” tells someone not to do something. Such a sentence takes “will you” as its tag.

Why the Other Options are Incorrect?

Option 1: The new teacher speaks Tamil fluently, don’t she? — Incorrect. The given sentence says “don’t she”. With he, she or it, the helping verb is “does”, not “do”.
Corrected: The new teacher speaks Tamil fluently, doesn’t she?

Option 2: We haven’t seen the new bridge yet, haven’t we? — Incorrect. The sentence is negative, but the tag is also negative. A negative sentence takes a positive tag.
Corrected: We haven’t seen the new bridge yet, have we?

Option 4: Meera was cooking rajma for dinner, wasn’t Meera? — Incorrect. The given sentence repeats the name “Meera” in the tag. A tag always uses a pronoun for the subject.
Corrected: Meera was cooking rajma for dinner, wasn’t she?