An action in a sentence can be represented in two ways, namely active and passive voice.
The passive voice always uses the past participle form of the main verb irrespective of any tense. Only the auxiliary verbs depend upon the sentence given in the active voice.
The correct answer is C)Let a glass of water be brought
Option C is correct as the object of the given sentence (a glass of water ) comes in the place of the subject (which is not required as it is understandable that the person whom the speaker is speaking to is standing in front of the speaker).
As the speaker asks the listener to do something,'let' is used, the given sentence in the simple present tense, and 'be' is used with the past participle form of the main verb.
Options A,B, and D are wrong as it is compulsory to use 'let' because the sentence is a command, 'should' cannot be used here, the given sentence is not in the future tense respectively.