A sugar 'X' dehydrates very slowly under acidic condition to give furfural which on further reaction with resorcinol gives the coloured product after sometime. Sugar 'X' is

When monosaccharides are heated with acids, dehydration occurs.

The product formed depends on the number of carbon atoms present in the sugar.

Pentoses dehydrate to form:

$$\mathrm{Furfural}$$

whereas hexoses form:

$$\mathrm{Hydroxymethylfurfural}$$

Since the reaction specifically gives furfural, sugar $$\mathrm{X}$$ must be a pentose.

Thus, tetroses are eliminated.

The question also states that dehydration occurs very slowly.

Ketoses undergo dehydration rapidly under acidic conditions, whereas aldoses dehydrate more slowly.

Hence, sugar $$\mathrm{X}$$ must be an aldose.

The furfural formed further reacts with resorcinol to produce a coloured product, confirming the dehydration reaction.

Therefore, sugar $$\mathrm{X}$$ is:

$$\mathrm{Aldopentose}$$

Correct option:

$$\mathrm{A}$$