Both a) and b) are non-idiomatic in English. c) is an option. I find the original sentence to be correct.
New words, one handy idiom, and a 2-minute quiz — delivered to your inbox to keep your streak alive.
Odessa DawnHi.Is it polite to say so using must?I have no idea what you are referring to. Did you intend to start your own post with a question including more context or details?