Which one is disperse dye?
(a) Congo red
(b) Alizarin
(c) Celliton
(d) None of these

Hint: As we know that disperse dye is a category of synthetic dye which is intended for polyester and related hydrophobic fibers. These dyes are polar molecules which contain anthraquinone or azo groups. It is also seen that upto 85% of dispersed dyes are azos or anthraquinone dyes.

Complete answer:
Let us first discuss about disperse dyes:-
-Disperse dyes are also called sublimation inks because the ink molecules tend to change directly from a solid to gas due to the application of heat. These dyes are small polar molecules that usually contain anthraquinone or azo groups which do not have charged cationic or anionic groups within the structure.
-These dyes are partially soluble in water and non-ionic in nature. Disperse dyes have better diffusion when boiled to a higher temperature.
Now let us check, which of the given dyes is a disperse dye:-
(a) Congo red: It is an organic compound which is soluble in water, yielding a red colloidal solution. Also its solubility is greater in case of organic solvents. But the use of congo red has been abandoned, primarily due to its carcinogenic properties.
(b) Alizarin: It is an organic substance which has been used throughout the history as a prominent red dye, principally for dyeing fabrics in the textile industry. Earlier this dye was derived from the roots of plants of the madder genus.
(c) Celliton: It is an organic compound, which is sparingly soluble in water and contains anthraquinone or azo groups which do not have charged cationic or anionic groups within the structure. Also it is blue in color and sold in powdered form. Therefore it is a dispersed dye.

Hence the correct answer is (c) Celliton

Note:
It is known that the light fastness and wash fastness of disperse dyes is generally good, but difficulty has been encountered in case of fume fading of certain disperse dyes. Also many disperse dyes have appreciable vapor pressures at higher temperature temperatures and can be dyed onto thermoplastic fibers by sublimation process.