Which of the following words is considered a helping verb?

"Will" is categorized as a helping verb because it assists the main verb in a sentence to express tense, mood, or voice. Helping verbs, also known as auxiliary verbs, work alongside the main verb to clarify the action or state of being. In this case, "will" helps form future tense constructions, as in "I will run."

The other words listed primarily function as main verbs that demonstrate action. "Run," "jump," and "sing" each describe a specific action performed by a subject and do not serve the grammatical function of providing additional contextual information regarding another verb. Thus, "will" stands out as the only helping verb among the options provided.