how to put back data together into new dataframe

Question

lets say I have original data frame as

a             b                    c          d
458          aas                   36         -5
320          def                   40         -0.4
009          zzz                   20         3
320          abh                   74          6

i then wrote some cote to group it df.groupby('a')['c'].max() I got this output

a                                         
009            20         
320            74          
458            36         

how to go back to displaying the whole date? expected output.

     a             b                    c          d
    009          zzz                   20         3
    320          abh                   74          6
    458          aas                   36         -5