If $X$ is a binary number which is power of $2$, then the value of $X \& (X-1)$ is :
X & X-1 is a bitwise operation which is used to check whether a number is power of 2 or not.
For ex. X= 8(1000), X= 7(0111)
X & X-1 = 0000.
Let X= 16(10000) , X= 15(01111)
X & X-1 = 00000.
The Expression X & X-1 will always give 0 when X is power of 2.
Hence option b) is correct.