The answer is A.
You can recognize the pattern of an exponential equation to know the answer, but to prove it, the easiest way is to inspect the elements and find exponential.
In this case, y = 2^x power
2^1 = 2
2^3 = 8
2^5 = 32
2&7 = 128
That is the best reason for the answer I can give sadly.
