Natural selection acts on variation by:

a. allowing the bad genes to quickly spread through the population

c. allowing the bad genes to slowly spread through the population

b. allowing the good genes to slowly spread through the population

d. allowing the good genes to quickly spread through the population

The answer is d. allowing the good genes to slowly spread through the population.

Natural selection is the differences in survival and reproduction as the consequence of differences in genotypes and, consequently, phenotypes. It is one of the factors of evolution that affects a frequency of different variations. In natural selection, genotype variations that will increase the chance of survival and reproduction of some organism are preserved and will be inherited Thus, "good genes" will be inherited. But, as the evolution, it will be slow process, since rarely evolution happens quickly.