In English, we often leave out "o'clock" - it's perfectly fine to say "It's seven" or "I'm leaving at three-thirty." Not so in French - you always have to say heure (except with midi and minuit)
A
Correct answer
Explanation
In English, we commonly say 'It's seven' or 'I'm leaving at three-thirty' without 'o'clock'. In French, you must include 'heure(s)' when stating time except with 'midi' (noon) and 'minuit' (midnight). For example, 'Il est sept heures' (It's seven o'clock) but 'Il est midi' (It's noon).