Which method can darken a deep red naturally without adding black?

Correct answer: Add a small amount of deep blue or violet

The governing concept is chromatic darkening rather than simply lowering value with black. A very small amount of deep blue or violet can shift red toward a richer, darker shadow while retaining colour character; the mixture should be tested gradually because too much will overpower the red. Option A is correct. White lightens, water mainly dilutes, and bright yellow usually makes the mixture lighter or warmer rather than naturally dark.