Which adjustment would make blue-green look more like natural foliage?

Correct answer: Increase green

The governing concept is changing a tertiary colour by altering the proportion of its components. Blue-green can be shifted toward a foliage-like green by increasing its green component, so option C is the best adjustment. Greatly increasing blue would make the colour more blue and less plant-like, violet would move it toward purple, and red would reduce the clean green impression or make the mixture duller.