The correct option is D Smoking
Option C is correct because the verb ‘stopped’ in the sentence needs an object and ‘smoking’ as a gerund serves as an object. Here, 'smoking' as a gerund indicates the act of smoking and hence, serves as an object to the verb 'stopped.' Option D and Option B cannot be used as it would render the sentence grammatically incorrect. Option A cannot be used because ‘to smoke’ can be used to indicate an activity in future and this sentence is talking about the activity in past.