Adding black makes a colour gradation muddy. What may be a better option for obtaining a darker value?

Correct answer: Use a stronger concentration of the same colour or a suitable related dark colour

The governing concept is preserving colour character while changing value. Instead of automatically adding black, the artist can use a more concentrated mixture of the same pigment or a compatible darker neighbouring colour. This usually produces a richer shadow and avoids a dull, muddy result. White raises the value, water generally dilutes it, and changing paper does not itself create the darker mixture, so B is correct.