1. Identify the test being described

   The question says:

   • sugar $X$ dehydrates very slowly under acidic conditions,
   • forms furfural,
   • which then reacts with resorcinol to give a coloured product.

   This is the basis of Seliwanoff-type differentiation and related furfural/hydroxymethylfurfural tests.

2. Recall dehydration behavior of sugars in acid

   Under strong acidic conditions:

   • Pentoses dehydrate to give furfural.
   • Hexoses generally give 5-hydroxymethylfurfural.
   • Ketoses dehydrate faster than aldoses.

   Since the product mentioned is specifically furfural, the sugar must be a pentose-type sugar.

3. Check each option

   Option A: Aldopentose

   • An aldopentose has 5 carbon atoms.
   • On acid dehydration, pentoses form furfural.
   • Being an aldose, the dehydration is relatively slower compared with ketoses.
   • This matches: dehydrates very slowly to give furfural.

   So, A is correct.

   Option B: Aldotetrose

   • A tetrose has 4 carbon atoms.
   • It does not typically give furfural on acid dehydration in the standard carbohydrate tests.
   • Hence this does not match.

   So, B is incorrect.

   Option C: Oxalic acid

   • Oxalic acid is not a sugar.
   • It cannot give furfural in this context.

   So, C is incorrect.

   Option D: Ketotetrose

   • A ketotetrose has 4 carbon atoms.
   • Tetroses do not give the characteristic furfural test described here.
   • Also, ketoses dehydrate faster, but the key issue is that furfural formation here is characteristic of pentoses.

   So, D is incorrect.

4. Conclusion

   The sugar $X$ must be an aldopentose.

$\boxed{\text{Aldopentose}}$