There is a slight misprint in the solution otherwise option D is correct
For a superkey you need to make sure that it contains one of the candidate key :
2m ways are there to pick up one of the m keys but it also includes the choice when none of them is choosen so we need to subtract that case (2m-1)
Rest of prime attributes may or may not be picked, So 2n-m ways.
By rule of counting:
Total ways : (2m-1).(2n-m )