Blue-green has become too green. Which colour should be increased to balance it?

Correct answer: Blue

The governing concept is restoring the balance between the component colours of a tertiary mixture. Blue-green is made from blue and green. If it has become too green, increasing blue brings the mixture back toward a balanced blue-green, so option C is correct. Yellow, red, or violet are not the original balancing component for this mixture; they would introduce another hue and could make the colour dull or change it significantly.