Which of these factors must exist for natural selection to cause change? a. a population must be stable. b. the ecosystem cannot contain predators. c. a population must have genetic variation. d. the ecosystem must have ample resources.

Answer; C
A population must have genetic variation; 

Explanation;
There are several factors that must be present for natural selection to occur in a population;;
First is the ability of a population to overproduce offspring, then heredity, variation in fitness or organisms, and also variation in individual characters among members of the populations.
Variation is important, since if there is no variation, there would be no survival of the fittest. This is because if all organisms , then the gene pool remains the same, and it makes no difference which ones are killed. 

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