Modals are used to show that we believe something is certain, probable or possible.

From the given options, Option A is correct.

We use the modal 'must' to show we are sure something to be true. It shows compulsion. The recruits in this case are being instructed that though they may grow their hair, it must never touch the collar.

Option B, C and D are incorrect as:

'Should' shows suggestion,

'Could' shows that something is possible in the future, but not certain,

and 'Would' is most commonly used to create conditional verb forms.