The correct answer is: A. A great environmental resistance.
Exponential growth is a type of population growth in which the population grows at a constant rate in a given period of time. This means that the population doubles in size every fixed period of time. Exponential growth can occur when there is a great environmental resistance, which is a factor that limits the growth of a population. Environmental resistance can include things like food availability, predation, and disease.
Option B, no environmental resistance, is not correct because exponential growth cannot occur without some form of environmental resistance. If there were no environmental resistance, the population would grow infinitely, which is not possible.
Option C, no biotic potential, is also not correct because biotic potential is the maximum rate at which a population can grow under ideal conditions. Even if there is no environmental resistance, the population cannot grow faster than its biotic potential.
Option D, a fixed carrying capacity, is not correct because exponential growth does not occur when there is a fixed carrying capacity. A carrying capacity is the maximum number of individuals that can be supported by a given environment. If there is a fixed carrying capacity, the population will eventually reach that carrying capacity and stop growing.
In conclusion, the correct answer is: A. A great environmental resistance.