Answer: A (D.) helping verb may be added to a main verb to make a verb phrase.
Explanation: Helping verbs accompany main verbs in order to add a particular meaning to a sentence. Furthermore, two types of helping verbs are auxiliary verbs and modal verbs. Some of the most common helping verbs are "be" and "have", which are used to form the progressive tenses and the perfect tenses. In that way, in the sentence "John is singing", "is" is the helping verb, while "singing" is the main verb.
