UPSC NDA I 2014 General Ability Paper

Option (a) "would" is more polite, because it expresses the idea of probability, and of willingness, and of the desire that something be done,
whereas option (b) "should" is the past tense of shall, but it is an auxiliary verb with a few uses, not all of which are in the past tense. It is used to ask question, to show obligation or to show a possible future event.
Option (c) 'must' is specifically used when an order is given or something is necessary to do.
The underlined word can be shows the capability of the subject to do an action.
Thus, from the above explanations, it is clear that option (a) is most appropriate in this sentence