Explanation

(d)
When a base accepts a proton, then an acid is formed. This acid is known as conjugated acid of that base. For example :
$NH_{3} + H^{+} \rightleftharpoons NH_{4}^{+}$
base conjugated acid
The conjugated acid of $HF_{2}^{-}\ is\ H_{2}F_{2}$.
$HF_{2}^{-} + \ H^{+} \rightleftharpoons H_{2}F_{2}$
base conjugated acid