Which option is most suitable for lowering green saturation while keeping its value nearly constant?

Correct answer: Add red of a similar value

The governing concept is neutralising saturation with a complementary colour while controlling value. Red is complementary to green; adding a small amount of red with a similar lightness reduces green’s chromatic intensity without greatly raising or lowering its value. Thus option A is correct. White raises value, black lowers value, and extra yellow shifts the hue toward yellow-green rather than neutralising it.