The laevorotatory isomer of 2-amino-3-hydroxypropanoic acid
A rule of thumb for determining $D / L$ isomeric form of an amino acid is the corn rule.
The groups: $\mathrm{COOH}, R, \mathrm{NH}_2$ and $\mathrm{H}$ (where $R$ is an unnamed carbon chain) are arranged around the chiral center carbon atom. Sighting from the hydrogen atom, if these groups are arranged counterclockwise around the carbon atom, then it is the D-form. If clockwise, it is the L-form. According to priority order : $-\mathrm{NH}_2,-\mathrm{COOH},-\mathrm{CH}_2 \mathrm{OH},-\mathrm{H}$.