There are 23 helpings verbs in the English language. All of those listed are technically helping verbs, although, if you were to circle one, the correct answer would be (d) be.
Helping verbs must be paired with another verb, such as "She must go to school." Must is a helping verb and it is paired with go, the verb.
Be, however, can be used in other ways as well, such as in the case of, "She will be there." Here, it as used as a verb, which allows you to circle (d) as your answer.