Answer:
b) the Ka of NHââş to find the hydronium concentration
Explanation:
The equilbrium of NHââş (The conjugate acid of NHâ, a weak base), is:
NHââş â NHâ + Hâş
Where Ka of the conjugate acid is:
Ka = [NHâ] [Hâş] / [NHââş]
Thus, if you know Ka of NHââş and its molar concentration you can calculate  [Hâş], the hydronium concentration, to find pH (Because pH =  -log [Hâş])
Thus, right option is: