Transform the statement 'He has been here before' into a question.

The transformation of the statement 'He has been here before' into a question requires maintaining the original meaning while altering its structure to fit that of an interrogative sentence. The correct transformation retains the original verb tense and phrasing, resulting in 'Has he been here before?'

This option effectively uses the present perfect tense, which is consistent with the original statement's tense. It directly turns the assertion into a question while keeping the intended inquiry about the person's past visits in place.

The other choices deviate in various ways. For instance, some alter the tense or the focus of the question altogether. Variations like 'Is he here already?' or 'Was he here once?' introduce different tenses or specify a single past visit rather than the possibility of multiple past visits, diverging from the original meaning.