Where can unwanted red influence enter a green mixture?

Correct answer: From a reddish yellow or reddish blue

The governing concept is pigment bias during subtractive mixing. Green is made by combining yellow and blue, but if either primary carries a reddish bias, that hidden red influence acts like the third primary and partially neutralises the green. Thus option A is correct and can produce a duller mixture. Green-biased pigments are more compatible, while white paper and clean water do not add red pigment to the paint.