Which type of yellow is considered relatively cool?

Correct answer: Green-biased yellow

The governing concept is hue bias on the colour wheel. Yellow that shifts toward green is relatively cooler than yellow that shifts toward orange or red, because green lies on the cooler side of the usual warm–cool comparison. Thus option C is correct. Red-biased and orange-biased yellows appear warmer, while brown-biased yellow is less clearly defined and does not provide the standard cool-yellow relationship.