This is true of the population which are included in Hardy-Weinberg equilibrium

The correct option is "mating is random".

The Hardy-Weinberg equilibrium is a fundamental principle in population genetics which states that allele and genotype frequencies in a population will remain constant from generation to generation in the absence of other evolutionary influences.
For a population to remain in Hardy-Weinberg equilibrium, five key underlying assumptions must be met:

1. Random Mating: Individuals must mate pair-wise at random with respect to their genotypes. If mating is non-random (e.g., assortative mating or inbreeding), it will alter genotype frequencies, causing deviations from the equilibrium.
2. No Mutation: No new alleles can be introduced into the gene pool through mutation.
3. No Gene Flow (Migration): There must be no movement of individuals or gametes into or out of the population, which would otherwise introduce or remove alleles.
4. Large Population Size: The population must be infinitely large (or sufficiently large) to minimize the effects of genetic drift, which is the random fluctuation of allele frequencies due to chance events.
5. No Natural Selection: All genotypes must have equal survival and reproductive success, meaning there is no selective pressure favoring one allele over another.

Let's evaluate the options against these assumptions:

- "entities migrate constantly": Incorrect, because migration (gene flow) introduces or removes alleles, disrupting the equilibrium.
- "populations should be limited and small": Incorrect, because small populations are highly susceptible to genetic drift, which changes allele frequencies by chance.
- "process of natural selection is occurring": Incorrect, because natural selection favors certain alleles, leading to a change in their frequency over generations.
- "mating is random": Correct, as random mating ensures that alleles are combined into genotypes purely by chance, satisfying one of the key requirements to maintain the equilibrium.