The tendency of a population to remain in genetic equilibrium may be disturbed by:

The correct option is A lack of random mating
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. This is called genetic equilibrium or Hardy-Weinberg equilibrium.

However, no population can remain in genetic equilibrium (Hardy-Weinberg equilibrium) indefinitely.

The allele frequencies change due to various factors like natural selection, gene flow/gene migration, mutations and lack of random mating, etc.

These changes in gene frequencies bring about evolution.

So, the correct answer is option D.