Correct option is D. $$15$$ days
$$(d) 15$$ days
Because,
A garrision of $$500$$ men had enough food for $$=24$$ days
$$1$$ men had enough food for $$=(24\times 500)$$
$$\therefore 800$$ men had enough food for $$=(24\times 500)/800$$
$$\Rightarrow15 days$$