D/L configuration is assigned to Fischer projection formula of monosaccharide on the basis of the lowest chiral carbon.
Threose has two chiral carbons C-2 and C-3.
The given Fischer projection formula of threose has OH groups at the lowest C -3 chiral carbon on the right side.
\(\therefore\) It is D-threose.
Ribose has three chiral carbons C – 2, C – 3 and C -4.
The given Fischer projection formula of ribose has – OH group at the lowest C -4 chiral carbon on the left side.
\(\therefore\) It is L-ribose
