Which mixture is more likely to produce a warm brown?

Correct answer: Red and green with more red

Brown can be obtained by mixing complementary colours or by combining several hues until the mixture is neutralised. A red-and-green mixture with red in greater proportion can produce a brown with a warm, reddish bias, so option B is correct. Blue and white, yellow and white, or green and white mainly create lighter tints of those colours rather than brown. The exact result depends on pigment strength, but the stated red dominance gives the intended warm direction.