Since the species is not reaching its maximum population, we can think that there is an inhibiting factor in the ecosystem. So if more organisms were to migrate within the population or if the birth rate were to increase, the population would not follow the trend in the image. If the resources of the ecosystem were limited, the population would increase, but then they would start to die due to lack of resources so the graph would have a gaussian bell distribution.
Therefore, the answer is D, since the maximum load of the ecosystem has been reached and although the organisms continue to reproduce, the population cannot increase but neither does it decrease since there are sufficient resources.