The correct answer is A. You mustn’t disturb my mother.
The sentence “My mother mustn’t be disturbed” is a negative imperative sentence. It is a command or request that something should not be done. In this case, the speaker is commanding or requesting that the listener not disturb their mother.
The other options are all incorrect because they do not express a negative imperative. Option B, “You shouldn’t disturb my mother,” is a modal verb construction that expresses a suggestion or recommendation. Option C, “You must be disturbing my mother,” is a present progressive construction that expresses a belief or suspicion. Option D, “You have disturbed my mother,” is a past participle construction that expresses a fact.
In conclusion, the correct answer is A. You mustn’t disturb my mother.