The correct answer is A. dN/dt = rN.
The formula for exponential population growth is dN/dt = rN, where dN/dt is the rate of change of the population, r is the rate of growth, and N is the population size.
Option B is incorrect because dt/dN is the rate of change of time, which is not the same as the rate of population growth.
Option C is incorrect because dN/rN is the rate of change of the population divided by the rate of growth, which is not the same as the rate of population growth.
Option D is incorrect because rN/dN is the rate of growth divided by the rate of change of the population, which is not the same as the rate of population growth.