What should be done to shift an orange mixture gradually from yellow-orange to red-orange?

Correct answer: Gradually increase red

The governing principle is controlled proportion in pigment mixing. Orange lies between yellow and red; increasing the red component step by step shifts the balance away from yellow-orange and toward red-orange. Therefore, option B is correct, and gradual additions produce a smooth progression instead of an abrupt colour change. White would make a tint, black would darken it, and blue would reduce the orange character or make it duller.