"The museum does not open on Mondays."
Translation:Il museo non apre i lunedì.
"Il museo non apre il lunedì"- I think this is the only correct answer. I'm not sure about "i lunedì", because in Italian when you want to say that something happens regularly on a certain day, you put "il" in front of the day of the week from Monday to Saturday, and "la" in front of domenica (Sunday). E' chiuso la domenica- It is closed on Sundays. Il Martedi faccio la spesa- I do my shopping on Tuesdays.