Which of the following is the most basic?

Ammonium hydroxide $(NH_4OH)$ is a:

Cis-butene dioic acid $\overset{K_{a_1}(-H^+)}{\longleftrightarrow} X_1^- \overset{K_{a_2}(-H^+)}{\longleftrightarrow} X_2^{2-}$
Trans-butene dioic acid $\overset{K'_{a_1}(-H^+)}{\longleftrightarrow} Y_1^- \overset{K'_{a_2}(-H^+)}{\longleftrightarrow} Y_2^{2-}$
The incorrect statement regarding the above information is:

The $pK_a$ value of the strongest acid among the following is:

Which one of the following substances has the highest proton affinity?

Which of the following is not a Lewis base?