Which of the following is an example of a helping verb?

A helping verb, also known as an auxiliary verb, is used alongside a main verb to indicate tense, mood, or voice. It helps to form different tenses or to express necessity or possibility. The word "was" is a form of the verb "to be," and it serves as a helping verb in constructions like "was running" or "was going," where it pairs with the main verb to convey actions that were occurring in the past.

The other options provided are main verbs in their base or past forms, referring directly to actions, but they do not serve the function of assisting another verb in this way. Thus, "was" stands out as the only helping verb among the choices given.