Which mixture is useful for creating a natural dark red in acrylic?

Correct answer: Mix a very small amount of green into red

Red and green are complementary colours, so a very small amount of green reduces the intensity of red and can create a deeper, more natural muted red. Option A is correct, provided the green is added gradually; too much would make the mixture brown or muddy. White would create a lighter tint, yellow would shift red toward orange, and water would mainly thin the paint rather than deepen its hue.