Answer:
HNO₂
Step-by-step explanation:
An acid is a proton donor; a base is a proton acceptor.
Thus, NO₂⁻ is the base, because it accepts a proton from the water.
H₂O is the acid, because it donates a proton to the nitrite ion.
The conjugate base is what's left after the acid has given up its proton.
The conjugate acid is what's formed when the base has accepted a proton.
NO₂⁻/HNO₂ make one conjugate acid/base pair, and H₂O/OH⁻ are the other conjugate acid/base pair.
NO₂⁻ + H₂O ⇌ HNO₂ + OH⁻
base acid conj. conj.
acid base