@Mukulvyas consider f(n) = 2n and g(n) = n
we can prove f(n) = O(g(n)) where 2n <= c n
where c(constant) is 3 and n0>1
But now to our problem 2^f(n) = O(2^g(n))
f(n) = 2n and g(n) =n
so 2^2n <= c 2^n
we can’t prove, it because 2^2n = 2^n *2^n
2^n *2^n <= c * 2^n
2^n <= c
cause c is constant it can’t be greater than variable 2^n
So,This option is incorrect.