If adding blue does not make green cool enough, what is the next controlled step?

Correct answer: Add a very small amount of more blue

The governing concept is controlled colour mixing: blue generally contributes a cool visual bias to green. If the first small addition is insufficient, add only a tiny further amount, mix thoroughly, and check a swatch before repeating. This makes option A correct. Red or extra yellow would usually push the mixture warmer, while much white mainly creates a lighter tint rather than reliably cooling the hue.