The word auxochrome is derived from the Greek words auxano (meaning increase) and chrom (meaning colour). This means that the auxochrome emphasizes the original colour of the dye.
The auxochrome itself does not produce colour, but if the group is coupled to the chromophore, it emphasizes the colour produced by the chromophore. Examples of
auxochromes are hydroxyl group (-OH), amino group (-NH₂), aldehyde group (-CHO) and methyl mercaptan group (-SCH₃).

If we consider an example; Benzene is colourless on its own, but when a nitro group is added to the benzene ring to create nitrobenzene, it takes on a light yellow hue. Therefore, the nitro group functions as a chromophore. Now that we've taken a closer look at the substance p-hydroxynitrobenzene, we can see that it has a rich yellow hue. As a result, the hydroxyl group here functions as an auxochrome.

Hence, the correct option is B.