What can be added in a very small amount to slightly reduce the warmth of red-orange?

Correct answer: A very small amount of blue

The governing concept is warm–cool colour adjustment. Red-orange is warm, whereas blue is cool, so a very small quantity of blue can moderate its warmth. Option B is correct because the quantity must be controlled; too much blue could shift the hue strongly toward a dull violet or brownish mixture. More red or yellow would increase warmth, and white would mainly lighten the colour.