(N/A) The dissociation of acetic acid is given by the equilibrium: $CH_{3}COOH_{(aq)} \rightleftharpoons H^{+}_{(aq)} + CH_{3}COO^{-}_{(aq)}$.
When $0.05 \ M$ acetate ions $(CH_{3}COO^{-})$ are added to the $0.05 \ M$ acetic acid $(CH_{3}COOH)$ solution,the concentration of the common ion,$CH_{3}COO^{-}$,increases.
According to Le Chatelier's principle,the increase in the concentration of the product $(CH_{3}COO^{-})$ causes the equilibrium to shift to the left.
As a result,the dissociation of $CH_{3}COOH$ is suppressed,leading to a decrease in the concentration of $H^{+}$ ions ($[H^{+}]$ decreases).
Consequently,the degree of dissociation of acetic acid decreases,and the $pH$ of the resulting solution increases.