We use modal verbs to show if we believe something is certain, probable or possible (or not). We also use modals to do things like talking about ability, asking permission making requests and offers, and so on.
In the given sentence, option A is correct as it signifies necessity,
compulsion and certainty. It fits in the context of a rule book given to new
recruits.
Option B and D are incorrect as they show that something is
possible in the future, but not certain.
Option C is incorrect because we use the modal 'can' to
make general statements about what is possible.