What is the best way to make a warm red relatively cooler?

Correct answer: Add a slight blue or violet bias

The governing principle is hue bias: a warm red usually leans toward yellow, whereas a cool red leans toward blue or violet. Adding a small amount of blue or violet shifts the red toward the cooler side without immediately destroying its red identity, so B is correct. Yellow and orange strengthen warmth. White changes lightness and may reduce intensity, but it does not reliably create a cool hue by itself. The adjustment should be gradual because too much blue or violet will produce purple.