Answer
$90^{\circ}$
Work Step by Step
Supplement of $90^{\circ}$ will be another angle that terminal side needs to sweep in order to reach at standard position of $180^{\circ}$. (Figure 24)
Obviously it will be $90^{\circ}$.
Another way to find supplement of an angle will be to subtract it from $180^{\circ}$.
Hence supplement of $90^{\circ}$ = $180^{\circ}$- $90^{\circ}$ = $90^{\circ}$
