What are some factors that might stop a population's exponential growth

So first of all, you might run out of food, which means that there wont be enough to sustain too many animals/people.
Or, you could simply run out of the space to have any more offspring. 
One final one is the introduction of a new predator, which would eat the population, making the growth unpredictable.

Hope this helped :D

Letter c i’d say