Which table represents an exponential function?

1. Which table represents an exponential function?
a.
x | 1 3 9 27
f(x) | 2 4 6 8
b.
x | 0 1 2 3
f(x) | 6 12 24 48
c.
x | 0 1 2 3
f(x) | 5 8 11 14
d.
x | 1 2 4 8
f(x) | 0 1 2 3