Final answer:
The population undergoing natural selection is not in Hardy-Weinberg equilibrium, as natural selection alters allele frequencies, which violates the conditions of the Hardy-Weinberg principle.
Step-by-step explanation:
The population that is not in Hardy-Weinberg equilibrium is d) A population undergoing natural selection. The Hardy-Weinberg principle states that allele and genotype frequencies in a population will remain constant from generation to generation in the absence of other evolutionary influences. These influences include natural selection, genetic drift, mutations, non-random mating, and gene flow.
Populations are in equilibrium when 1) no new mutations are occurring, 2) there is no migration (gene flow), 3) the population is very large (to minimize genetic drift), 4) mating is random, and 5) there is no natural selection. Each factor can disrupt equilibrium by altering allele frequencies or genotype ratios in a population.