Aggregate Functions

In pandas, \texttt{max()}, \texttt{std()}, and \texttt{var()} are valid aggregate functions that can be applied to a DataFrame. \texttt{max()} returns the maximum value, \texttt{std()} computes the standard deviation, and \texttt{var()} computes the variance of the columns in the DataFrame. However, \texttt{sort()} is not an aggregate function; it is used to sort data in a DataFrame. Therefore, the correct answer is \texttt{sort()}.