Which method is suitable for making a cool blue relatively warmer?

Correct answer: Increase a slight violet or red influence

Relative temperature depends on the hue direction of a colour, not merely on how dark or muted it is. Blue becomes relatively warmer when a small red or violet bias is introduced, because those hues lie toward the warmer side of blue’s range. Option B is correct. Green shifts blue toward a cooler, cyan-like direction, while black and grey mainly alter value or intensity rather than supplying a warm hue bias. Thus A, C, and D do not achieve the stated purpose.