Correct Answer - A
The `C_(4)` sugars with the different chiral centres exist in four enantiomerc forms and as two racemic mixtures. These are known as threose and erythrose and have Fisher projections `I-IV`
Erythrose exists in two enantiomeric forms `(I` and`II)` as does threose `(III` and `IV`). The compounds represented by `I` and `III` (or `IV`) are diastereoisomers. They are stereoismers which are not enantiomeric.
