What three conditions must be present in order for natural selection to "work"?

a) Variation in traits within a population, heritability of those traits, and differential reproductive success based on those traits.
b) High mutation rates, genetic drift, and gene flow between populations.
c) Uniformity in traits within a population, rapid adaptation to changing environments, and high levels of genetic diversity.
d) Random mating, equal access to resources, and constant population size.