Why 'mustn't' is possible here is a mystery to me. 'Mustn't' means 'can't' or 'not allowed to'. Could you tell me if it is ok to say 'You mustn't worry about redundancies'?
Just don't think all of the options must mean the same. "Mustn't worry", which I'd describe as a perfect set phrase, means "don't worry" or "it's wrong to worry about redundancies", whereas "you needn't worry" or "don't have to worry" mean it's not necessary to worry about them.