Select the most appropriate option to fill in the blank and make a grammatically correct sentence.

Our neighbours threatened to call the police if we ________ stop the noise.

didn’t

The most appropriate option to fill in the blank is didn't.

The sentence is in the past tense, and the verb "stop" needs a past tense form. The past tense form of "stop" is "stopped." The negative form of "stop" is "didn't stop."

The other options are not as appropriate.

• Hasn't is the present perfect tense form of the verb "stop." This does not make sense in the context of the sentence, because the neighbors threatened to call the police if we didn't stop the noise in the past.
• Don't is the present tense form of the verb "stop." This does not make sense in the context of the sentence, because the neighbors threatened to call the police if we didn't stop the noise in the past.
• Haven't is the past participle form of the verb "stop." This does not make sense in the context of the sentence, because the neighbors threatened to call the police if we didn't stop the noise in the past.