What should be done to make red-orange more reddish?

Correct answer: Increase the amount of red

The governing concept is colour mixing by proportion. Red-orange is produced from red and orange, and its appearance can be shifted toward one component by increasing that component. Therefore, adding a larger amount of red makes red the dominant influence and makes the mixture look more reddish. Increasing blue or green would introduce a different colour bias, while removing white does not specifically add red.