A$.$ The groupby$()$ function splits a DataFrame into groups based on the specified keys and applies a function to each group.

B$.$ The groupby$()$ function can only group data by column names and cannot accept custom functions for grouping.

C$.$ The size$()$ function returns the number of non$-$null values in each group.

D$.$ The count$()$ function returns the number of non$-$null values in each group.

E$.$ Custom aggregation functions can be passed to the agg$()$ method to apply specific operations on the grouped data.

F$.$ The result of a groupby$()$ operation is always a DataFrame.

G$.$ The groupby$()$ function allows grouping data by the index $($rows$)$ or by columns.

H$.$ The groupby$()$ function automatically removes missing values from the group keys by default.