Which mixture can create a light warm brown?

Correct answer: Add white to a warm brown

The governing concept is tinting a prepared colour. A warm brown already contains a warm, earthy hue; adding white increases its lightness and produces a lighter warm-brown tint. Therefore option A is the best answer. Blue with black becomes a dark blue shade, blue-green remains in the cool family, and yellow added to violet tends to neutralise or shift it rather than reliably create a light warm brown.