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

(a) Aldopentose

Explanation:

• The sugar 'X' described in the question dehydrates under acidic conditions to produce furfural. Furfural is a compound commonly formed from pentoses (five-carbon sugars) when they undergo dehydration in acidic conditions.
• Aldopentoses are a type of pentose sugars with an aldehyde group (-CHO) and include sugars like ribose and arabinose. They are known to dehydrate and form furfural under acidic conditions.
• When furfural reacts with resorcinol, it forms a colored product, which is consistent with the behavior described in the question.

Why other options are incorrect:

• Aldotetrose (b): Tetrose sugars (four-carbon sugars) do not form furfural; they form different derivatives.
• Oxalic acid (c): Oxalic acid is not a sugar and does not form furfural.
• Ketotetrose (d): Ketotetroses (four-carbon sugars with a ketone group) also do not dehydrate to form furfural.

Thus, the sugar 'X' is an aldopentose.