a$.$ AddRoundKey b$.$ The MixColumn step, because this is where the different bytes interact with each other. c$.$ The ByteSub step, because it contributes nonlinearity to AES. d$.$ The ShiftRow step, because it permutes the bytes. e$.$ There is no wholesale swapping of rows or columns. AES does not require this step because: The MixColumn step causes every byte in a column to alter every other byte in the column, so there is not need to swap rows; The ShiftRow step moves bytes from one column to another, so there is no need to swap columns