Modals are used to show that we believe something is certain, probable or possible.
From the given options, option A is correct as it signifies certainty. The recruits are instructed to keep their uniform clean and tidy. After reading the sentence, we can make out that we need to use a modal that implies certainty.
Option B, C and D are incorrect as :
'Could' and 'Might' imply something is possible in the future, but not certain,
and 'Would' implies condition.