Draw the fischer projections of the four aldotetroses. draw the d sugar on the left and its l isomer directly to the right of it. switch between an aldose and a ketose by

An Aldotetrose is a tetrose sugar (4 Carbons) with an aldehyde functional group on the first atom.
The D- compound contains the last bottom asymmetric -OH group at the right side of the Fischer's projection. The L-compound contains the last bottom asymmetric -OH at the left side of the Fischer projection.
So the Fischer's projection of the four aldotetroses is as follow: