Something that controls the growth or size of a population is
a. the carrying capacity.
b. the growth rate.
c. a limiting factor.
d. a growth factor.

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The answer to the following question: Something that controls the growth or the size of a population is a limiting factor. The answer is the letter  

C. a limiting factor

I hope it helps, Regards.      

Demographically speaking, a limiting factor is the responsible for avoiding the regular functioning of the vegetal growth rate. Geopolitically speaking, we could be standing in front of a clear anti-birth policy, such as the one carried out in China through decades. Specifically, these kind of policies are based on non-financially stimulating decisions over the population (for instance, second family kids are not legally allowed to make use of the public educational service).