Which mixture can create a warm brown colour?

Correct answer: By adding a little blue to red-orange

The governing concept is neutralising a warm colour with a small amount of its complementary or balancing colour. Red-orange is strongly warm; adding a little blue reduces its intensity and can produce a warm brown, especially when the blue is added carefully. Thus option A is correct. White added to blue or violet makes tints, while extra blue in green produces a cooler green rather than brown.