Which mixture is suitable for darkening bright yellow while preserving its warm character?

Correct answer: Add a little orange or warm brown

The governing concept is colour temperature during value adjustment. A small amount of orange or warm brown lowers the brightness of yellow while keeping it within the warm colour family, so option A is correct. Blue would cool and possibly neutralise the yellow, black can make it dull or muddy when overused, and white increases lightness rather than darkening it. The mixture should be added gradually and tested before covering the whole area.