What precaution should a student take to obtain a clean orange?

Correct answer: Mix red and yellow with a clean brush

The governing concept is clean pigment mixing. Orange is made by combining red and yellow, and a clean brush prevents traces of blue, green, black, or another colour from contaminating the mixture. Such contamination can make orange dull or muddy. Thus option B gives both the correct pair and the necessary practical precaution. Adding blue, all three primaries, or extra black moves the mixture away from a clear orange.