If this -OH is on the right , it's the D enantiomer. The aldehyde function is at the top. We have put both OH groups on the left. O is on the left, so this is the L enantiomer. Let's also draw the D enantiomer.
Enantiomers are mirror image stereoisomers.
We put both OH on the same side. Diastereomers are non-mirror image stereoisomers . The rule for calculating the maximum possible number of stereoisomers is 2 n , where n is the number of chiral carbons. I and II are enantiomers of each other, L and D Erythrose. III and IV are enantiomers of each other, L and D Threose.