Answer: B, but technically, all of them are correct except C.
Explanation:
First, you fill in 1 for n:
= 2 x 4(1)
= 2 x 4
= 8
Next, I filled in 4 for n:
= 2 x 4(4)
= 2 x 16
= 32
Then, I filled in 8 for n:
= 2 x 4(8)
= 2 x 32
= 64
You can already figure this one out, and the answer is B.