Ответ:
\[y = x^{4} - четная.\]

\[81 = ( - 3)^{4};\ \ \ \ 81 = 81 \Longrightarrow A \in y.\]
\[125 = ( - 5)^{4};\ \ \ \ \]
\[125 \neq 625 \Longrightarrow B \notin y.\]
\[16 = 2^{4};\ \ \ \ 16 = 16 \Longrightarrow C \in y.\]
\[y = x^{4} - четная.\]

\[81 = ( - 3)^{4};\ \ \ \ 81 = 81 \Longrightarrow A \in y.\]
\[125 = ( - 5)^{4};\ \ \ \ \]
\[125 \neq 625 \Longrightarrow B \notin y.\]
\[16 = 2^{4};\ \ \ \ 16 = 16 \Longrightarrow C \in y.\]