This can also be solved by setting up a direct proportion. The full food supply serves 450 students, which is the same as 270 cops, so the ratio 450 students : 270 cops describes the whole stock of food. After 300 students eat, the remaining food would have served \( 450 - 300 = 150 \) more students, so we can form the proportion \( 450 : 270 :: 150 : x \), where \( x \) is the number of cops the remaining food can feed. Cross-multiplying gives \( 450x = 270 \times 150 = 40500 \), so \( x = 90 \). Let's verify each option using this cross-multiplication.
Only \( x = 90 \) satisfies the cross-multiplied proportion \( 450 : 270 :: 150 : x \) built from the food supply's fixed capacity.
Therefore, the correct answer is 90.