Which mixture can be useful for creating an earthy brown colour?

Correct answer: Red and green in a suitable ratio

The governing concept is complementary or near-complementary mixing. Red and green lie opposite each other on the traditional colour wheel; when mixed in a controlled ratio, they reduce one another’s saturation and can produce a neutral, earthy brown. Thus option D is correct. Blue and white make a light blue tint, yellow and white make pale yellow, and green with white makes a light green tint.