Answer
(b) $NH_3$, because it is the stronger base on the list.
Work Step by Step
Stronger bases can help the acid to go further toward the completition of the reaction, because they are more capable of attracting the proton from the acid.
In this list we have:
$H_2O$: $Kb = 10^{-14}$. (Very weak)
$Cl^-$: Conjugate base of $HCl$ (strong acid), therefore it is very weak.
$N{O_3}^-$: Conjugate base of $HNO_3$ (strong acid), therefore it is very weak.
So, ammonia ($NH_3$) is the strongest base in this list.
