The correct option is B Would
Modal verbs help determine the mood of a sentence, to indicate obligation, permission, possibility and so on. The speaker in the given sentence seems to make a request, a request to switch off the light; 'would' in this context also expresses a request, hence option B is correct. 'Will' refers to a future action, and doesn't sound polite when used here, hence option D is incorrect. 'Should' is used as the past tense of 'shall' in the Indirect speech but the given sentence is in direct speech. 'Shall' when used in the second person (you) indicates a threat or promise. This sentence expresses neither, also it isn't in indirect speech, hence options A and C are incorrect.