A mutation occurs when a change in the structure of a gene takes place, resulting in a variant of this gene, so this creates genetic variation.
Genetic recombination occurs when a gene suffers a sequence of breaks and rejoining of chromosomes or chromosome segments. This also creates a variant of this gene, so this creates genetic variation as well.
Sexual reproduction demands two gamete cells to fuse, creating a new cell that is a combination of those two cells. This creates a cell with a different genetic material, so this also creates genetic variation.
Therefore, the correct answer is D. all of the above