Taking into account the context of the given passage, it can be inferred that the selected option is the correct answer. Seeing that it is from a rule-book, the correct modal is option A, must. It imposes compulsion required in the context. The modals of options B,C and D all aim at some kind of possibility, probability and free will to exercise their wishes, whereas the given context would requires compulsory action and thus, the use of the modal must is correct.