The right answer is exponential.
In mathematics, economics and biology, we speak of an exponential (or geometric) growth phenomenon when the growth in absolute value of the population is proportional to the existing population, ie when the growth rate intrinsic is constant and positive.
When the phenomenon of exponential growth is continuous, we can model it by means of an exponential function: Nt = N0 exp (r.t)
N (t) is the number of individuals at time t, N (0) the number of individuals at time 0, r the intrinsic growth rate and t time, exp () being the Euler constant of approximately 2.718 .