Answer
The value multiplies by factor $b$ every $k$ units of time (e.g., $f(t) = 100(2)^{t/3}$ doubles every 3 hours).
Explanation
Digital SAT questions frequently ask to convert annual growth rates to monthly rates: $(1 + r)^t = ((1 + r)^{1/12})^{12t}$, where the monthly growth factor is $(1 + r)^{1/12}$.