In aldotetroses, if the two hydroxyl groups are on the same side in the Fischer projection, the sugar is named erythrose. If the hydroxyl groups are on opposite sides, it is named threose. The D and L designation is determined by the orientation of the -OH group on the chiral carbon furthest from the aldehyde group (C3 in this case). If the -OH group is on the right, it is a D-sugar; if it is on the left, it is an L-sugar. Although the diagrams are missing in the prompt, the standard sequence of structures given in this NEET 2016 question perfectly corresponds to D-erythrose, D-threose, L-erythrose, and L-threose.