What is the best choice to complete the statement: John F. Kennedy was a senator before he _______ president in 1960.

The correct choice accurately completes the sentence by using the passive voice, which is appropriate for this context. "Was elected" indicates that John F. Kennedy did not perform the action of election himself but was the recipient of the action—he was elected by others. This structure also aligns well with the past tense used in the rest of the sentence, as it clearly indicates that the election occurred in 1960, after his time as a senator.

The passive construction emphasizes the event rather than the individual’s action, which is fitting when discussing historical figures and their accomplishments. This choice maintains the flow of the sentence and ensures grammatical correctness in terms of verb forms and tenses.