Select the most appropriate option to improve the bold part of the sentence.
You must refrain to smoke.

Correct option is D

The correct substitution of the highlighted words is (d) refrain from smoking.
Explanation: The verb refrain is followed by the preposition from and a gerund (-ing form): refrain from + V-ing. “Refrain to smoke” is ungrammatical. Therefore, “refrain from smoking” is the idiomatic and grammatically correct form.
Grammatical rule used: Certain verbs require fixed prepositions and gerund complements: refrain from + V-ing, insist on + V-ing, succeed in + V-ing, etc.
Example: Students were advised to refrain from talking during the exam.
Information booster / exceptions:
· Contrast with stop: stop smoking (cease the activity) vs stop to smoke (pause another activity in order to smoke). Options (b) and (c) misuse this: stop to smoke changes meaning; stop of is ungrammatical.
· “Quit from smoke” is incorrect; quit smoking is the idiomatic form.