What is the most controlled way to make a cool blue relatively warmer?

Correct answer: Add a small amount of violet or red

Colour temperature depends on the direction in which a hue leans on the colour wheel. A cool blue can be shifted relatively warmer by adding a very small amount of violet or red, because both introduce a warmer bias. The quantity must be controlled so that the blue identity is not lost. Green generally pushes blue toward a cooler or greenish direction, while white and black mainly alter value, not temperature. Thus A is the best answer.