What can be added to make pink slightly darker?

Correct answer: A little red

The governing concept is changing a tint by increasing the amount of its base hue. Pink is a light tint of red, so adding a little red increases pigment strength and makes the pink darker and redder; option C is correct. White would make it paler, yellow would shift it toward a warm orange-pink, and green could dull or neutralise it. A small addition is important because too much red would remove the pink quality.