Which method is most suitable for darkening red-orange without greatly reducing its saturation?

Correct answer: Add a small amount of dark red

To lower value while preserving the hue family, add a compatible dark red in a small amount and test gradually. Blue-green is complementary and can neutralize the red-orange, reducing saturation; grey also dulls it, while white lightens rather than darkens it. Thus option A is the most suitable controlled adjustment.