The correct choice from the options provided is:
Natural selection can only affect traits present in a population.
This statement reflects the idea that natural selection acts on existing variations within a population, favoring certain traits that confer advantages for survival and reproduction. On the other hand, artificial selection is a process where humans selectively breed individuals with desirable traits, which can lead to the development of new traits (in a sense, applying direct influence over the selection process).
Other options contain inaccuracies regarding the nature of artificial and natural selection. Artificial selection is not random, and while both forms of selection may have unpredictable outcomes, this is not a key difference between the two.