According to Malthus, the population of a country grows geometrically. This means that the population grows at a rate that is proportional to the current population. This can be expressed mathematically as $P(t) = P_0 r^t$, where $P(t)$ is the population at time $t$, $P_0$ is the initial population, $r$ is the growth rate, and $t$ is time.
The geometric growth model is often used to model the growth of populations, such as the human population. However, it is important to note that the model is not always accurate. For example, the model does not take into account factors such as death rates, which can slow down the growth of a population.
The other options are incorrect because they do not accurately describe the way that the population of a country grows. Option A, arithmetically, is incorrect because the population does not grow at a constant rate. Option B, algebraically, is incorrect because the population does not grow at a rate that is proportional to the square of the current population. Option D, at constant rate, is incorrect because the population does not grow at a constant rate.