Identify the number of structure/s from the following which can be correlated to D-glyceraldehyde.

In carbohydrate chemistry we say that a molecule “correlates to” (or belongs to) D-glyceraldehyde when, in its Fischer projection, the configuration of the chiral carbon farthest from the carbonyl group is the same as that of D-glyceraldehyde (i.e. the -OH lies on the right).

D-glyceraldehyde Fischer projection:
  CHO
HO-CH
  CH2OH

Thus in any aldose or ketose:

• Locate the carbonyl carbon (C-1 in aldoses, C-2 in ketoses).
• Move down the chain to the last stereogenic (chiral) carbon (penultimate carbon in aldoses, the one just before the terminal -CH2OH).
• If the -OH on this carbon points to the right, the sugar is a D-sugar and therefore correlates with D-glyceraldehyde; if it points to the left it is an L-sugar and correlates with L-glyceraldehyde.

Now inspect the four structures given in the question:

Case I: Aldopentose with the -OH on the penultimate (C-4) carbon on the right ⇒ D-ribose (D-series) ⇒ correlates.
Case II: Aldohexose with -OH on penultimate (C-5) carbon on the left ⇒ L-glucose (L-series) ⇒ does not correlate.
Case III: Ketohexose (e.g. D-fructose). Carbonyl at C-2; the last chiral carbon is C-5. -OH on C-5 is on the right ⇒ D-fructose ⇒ correlates.
Case IV: Aldotetrose with -OH on penultimate (C-3) carbon on the right ⇒ D-erythrose ⇒ correlates.

Hence 3 of the 4 structures are D-sugars and therefore correlate to D-glyceraldehyde.

Option D which is: three