If yellow-green appears too blue what is the most controlled correction?

Correct answer: Add a small amount of yellow

The governing concept is controlled hue correction. Yellow-green contains yellow and green, so if the mixture looks too blue, a very small addition of yellow shifts the hue toward the intended yellow-green. Red may neutralise or muddy the mixture, while white mainly increases lightness and black decreases lightness. Therefore, option A makes the most precise correction without unnecessarily changing value or saturation.