John Kennedy was a senator before he _______ President in 1960.

The correct choice, "was elected," fits seamlessly into the sentence by providing the appropriate passive voice structure. The sentence emphasizes the action of John Kennedy being elected rather than focusing on him taking an active role in the election process. Using the passive form "was elected" correctly indicates that the action of electing him was performed by others, which aligns with the context of an election.

In this case, the past tense "was elected" reflects that the election occurred in the past, specifically in 1960. This complements the timeline of Kennedy's career and maintains clarity in describing his professional progression from being a senator to becoming president.

Other options do not maintain the required passive voice or proper tense for this context. For instance, "was electing" suggests an ongoing action that does not fit the context of a completed event, while "elected" and "had elected" improperly omit the necessary auxiliary verb to indicate that he was the recipient of the action rather than the one performing it. Thus, "was elected" is the only choice that correctly completes the sentence.