Explanation

(a)
The structure of D-glyceraldehyde is as
[image:12_image400.emf]
The priority of groups is decided by the following rules
(i) Atom having higher atomic number gets higher priority
(ii) If the priority cannot be decided by rule 1 then the next atoms are considered for priority assignment.
(ii) Where there is a = bond or=bond both atoms are considered to be duplicated or triplicated [image:12_image401.emf] has higher priority than $–CH_{2}OH).$ Hence, the correct order of priority of groups in D-glyceraldehyde is as:
$OH(1),\ CHO(2),\ CH_{2}OH\ (3)$and $H(4)$