A geometric sequence begins with 2, -8, 32, −128, 512, ...

Which option below represents the formula for the sequence?
a. f(n)=4(-2)ⁿ
b. f(n)=4(-2)ⁿ⁻¹
c. f(n)=2(-4)ⁿ⁻¹
d. f(n)=2(-4)ⁿ