These tables of values represent functions. in which table do the values represent and exponential function?

a. x y
1 5
2 9
3 13
4 17
5 21

b. x y
1 12
2 13
3 14
4 15
5 16

c. x y
1 16
2 29
3 42
4 55
5 68

d. x y
1 5
2 10
3 30
4 40
5 80