The correct option is D natural selection is operating in the population
The collection of all genes and their alleles represents the gene pool of a population.
Using Mendel's principles of inheritance, Hardy and Weinberg stated that the frequencies of the alleles tend to be stable and remain constant from generation to generation in sexually reproducing populations under certain conditions. Hence, the gene pool of a population remains constant under certain conditions. This is called genetic equilibrium or Hardy-Weinberg equilibrium.
The conditions include
(a) The size of the population should be large.
In a large population, there would be many alleles. In such a case, the loss of alleles due to any reason would be less pronounced. In due course of time, the allele frequency can be recovered as well. But in a small population, the loss of any allele would be more pronounced as there are a small number of individuals.
Hence, genetic equilibrium is easier to be maintained in a large population.
(b) No mutations in the members of the population.
Mutation refers to a change in the genetic material of an organism.
Mutations bring change in gene frequencies as new alleles may be introduced in the population. Hence, mutations can disturb the Hardy-Weinberg equilibrium.
(c) No gene migration or gene flow
The introduction of genes from one population to another is called gene migration or gene flow. Individuals or gametes (in the form of pollen) that enter a population may introduce new alleles in the population which may alter the gene frequency of the receiving population. Hence, gene migration can disturb the genetic equilibrium of the population.
(d) Natural selection is NOT operating in the population
Natural selection is a mode of evolution in which organisms which are better adapted to their environmental conditions, survive. The individuals that fail to cope with the changing environment perishes. If natural selection is operating in the population, it certainly disturbs the genetic equilibrium of that population.
No population can remain in genetic equilibrium (Hardy-Weinberg equilibrium) indefinitely. The gene frequencies change due to various factors like natural selection, gene flow, mutations etc. These changes in gene frequencies bring about evolution. Hardy Weinberg populations are hypothetical and never exist.