What should be done to shift yellow-orange more toward orange?

Correct answer: Increase the amount of orange

The governing concept is controlled colour mixing: increasing the proportion of one component shifts a mixture toward that component. Yellow-orange is made from yellow and orange. To move it further toward orange, increase the amount of orange, so option B is correct. More yellow would produce a yellower result, while blue or green would alter the hue and may reduce the clarity of the intended yellow-orange mixture.