Mohan and Shankar promised . . . . . . . . hard.

The correct answer is A. to work.

The sentence “Mohan and Shankar promised to work hard” is in the simple present tense. In the simple present tense, we use the infinitive form of the verb after modal verbs like “promise”. The infinitive form is the basic form of the verb, without any “to”.

The other options are incorrect because they are not in the infinitive form. Option B, “working”, is the present participle form of the verb. Option C, “worked”, is the past tense form of the verb. Option D is not a verb form at all.