Correct option is (a) 4
Easy explanation: The powers of four follow the given order:
4^1 = 4
4^2 = 16
4^3 = 64
4^4 = 256
4^5 = 1024 and so on.
Odd powers of 4, have the number 4 in the units place. When 5 divides the nearest ten, 4 will be obtained as the remainder each time.