Tell the truth. NO excuses, NO lies. You don’t need to go into detail about anything. There are a couple of very good ideas for exactly what to say in the responses above. They fit with the idea of being completely honest.
I think this is quite an important point actually. It might not seem like a big deal in early sobriety but it’s my view that honesty makes sobriety far easier. Alcoholics often have a hard time with the truth. I did. I think that we want the world to be “better” in a thousand different ways, so we play “pretend” . We drink alcohol to feel better about the things we don’t like. Then, eventually, we pretend that we can handle our drinking.
Ironically it’s only when really bad things happen, as a result of all this pretending, that life without alcohol begins to look more attractive. It’s my view that deceiving, or even misleading others, is a step back toward "the land of make believe” that ended up so troublesome in the first place.