$3.21$ You want to compute the mean of each group in a DataFrame group by a column $$Category$.$ Which combination of methods will you use?

a$.$ Df$.$groupby$($Category$).$mean$()$

b$.$ Df$.$groupby$(`$Category$`).$agg$(`$mean$`)$

c$.$ Df$.$grougby$(`$Category$`).$apply$(`$mean$`)$

d$.$ Df$.$grougby$(`$Category$`).$transform$(`$mean