The correct option is C Should
Modal verbs help determine the mood of a sentence, to indicate obligation, permission, possibility and so on. When 'should' is placed in this sentence, it expresses advice, which is relevant to the given sentence; the speaker is advising the listener "you" not to make hasty decisions, hence option B is correct. 'Shall' expresses a command here, but the sentence implies an advice, hence 'should' is preferable and option A is incorrect. 'Will' and 'would' refer to a future action ('would' is the past tense of 'will' in indirect speech), the given sentence does not indicate the future tense, but is more suitable as an advice; also, it isn't in indirect speech; hence options C and D are incorrect.