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

Correct answer: Increase the influence of red or orange

The governing concept is hue movement through changing component proportions. Yellow-orange lies nearer yellow, whereas red-orange lies nearer red on the colour wheel. Increasing the red influence, or using a warmer orange mixture with more red, moves the hue toward red-orange. Option A is correct. Blue or green would push the mixture toward cooler or duller directions, and grey mainly reduces saturation without creating the intended red shift.