Which of the following sentences demonstrates the active voice?

Active voice is characterized by a sentence structure where the subject performs the action expressed by the verb. In this case, "He ran a marathon last week" clearly shows the subject "He" taking action by running. The clarity and directness of active voice enhance the sentence's impact, making it more engaging for the reader or listener.

In contrast, the other sentences illustrate passive constructions, where the subject is receiving the action rather than performing it. "The marathon was run by him last week" emphasizes the marathon instead of the runner, which dilutes the immediacy of the action. "A marathon was run last week" also places focus on the marathon, lacking a direct subject-action relationship. Lastly, "There was a race" is vague and doesn’t indicate who participated or what occurred actively, which does not fulfill the criteria for active voice. Thus, the first sentence stands out as the best example of active voice.