Why can mixing blue-green with red-orange produce a more neutral colour?

Correct answer: Together, they bring the influence of all three primary colours

In a traditional subtractive colour model, red-orange and blue-green are complementary hues. Their pigment mixtures bring together red, yellow, and blue influences, which tend to neutralise one another and reduce saturation. The exact mixture depends on the pigments, but option A gives the relevant reason.