Answer
The conjugate base of $HNO_2 $ is $NO{_2}^-$.
Work Step by Step
To find the conjugate base of an acid, just remove one proton $(H^+)$ from the compound.
1. Remove one hydrogen:
$HNO_2 -- \gt NO_2$
2. Subtract one from the charge
$0 --\gt -1$
$NO_2 -- \gt NO{_2}^-$