The correct option is B to have
The infinitive verb is a verb with the word 'to' in front of it. The verb is always just a verb. There is no - ed, no - ing, no - s at the end of the verb. Thus, option A, C and D are wrong according to the rule. The correct option is B since 'to' is in front of the verb 'have'. Hence, sentence would become : He seemed to have finished his homework.