Final answer:
The sentence with an example of active voice is 'Cynthia was running a marathon last Saturday,' where the subject 'Cynthia' is performing the action.
Step-by-step explanation:
The sentence that is an example of active voice among the options provided is:
C) Cynthia was running a marathon last Saturday.
In this sentence, 'Cynthia' is the subject and she is performing the action of running, which makes it an active voice construction. In contrast, sentences A, B, and D use the passive voice, where the action of the marathon being run is something that happens to the subject, as indicated by the use of the passive construction 'was run by Cynthia.' Active voice is clearer and more direct because the subject of the sentence performs the action of the verb, resulting in stronger sentence construction.