1.2, 3, 7.5, 18.75, ... which formula can be used to describe the sequence?

1.2, 3, 7.5, 18.75, …

Which formula can be used to describe the sequence?

a. f(x) = 1.2(2.5)x – 1
b. f(x) = 2.5(1.2)x – 1
c. f(x) = 1.2(2.5)x
d. f(x) = 2.5(1.2)x