Mustn't.
The contraction of "must not" is spelled mustn't (a prohibition, only used in present tenses).As with other contractions of "not", the apostrophe replaces the missing O.
The contraction form of who is is who's.Example: Who's at the door?
haven't
you're
I've
They're
The contraction mustn't means "must not."
The contraction of "must not" is spelt mustn't.
The contraction "mustn't" means "must not." Must is an auxiliary verb and not is an adverb.
Shouldn't
The adjective form of who is spelled whose. The spelling of the contraction is who's meaning "who is."
The contraction is spelled shouldn't.