In the absence of mutation, migration and selection, if gene frequencies are changed in a population, the change is due to

The correct option is A Sampling error
The presence of multiples of complete haploid chromosomal sets is called as polyploidy.

Hybridization is crossing of two pure-breeding varieties to produce new combinations of genes in the progeny. Crossing over is the process of exchange of genetic material between non-sister chromatids of homologous chromosomes to produce new genetics combinations or variations. Polyploidy, crossing over and hybridization produces new combinations of already existing genetic material/genes but does not introduce a new allele or delete an existing allele from population gene pool. Thus, they do not change genetic frequencies in Hardy-Weinberg population.

Sampling errors refer to changes in allelic frequencies from one generation to the next due to an incorrect sampling of the alleles of the parent generation the small populations are more prone to sampling error.

Thus, the correct answer is option C.