Combining the genes of two parents in offspring increases in the population, which is valuable for a species because it provides more characteristics for natural selection to operate on.

The correct answer will be- variation

Explanation:

Natural selection is the process in which an organism becomes adaptable to its environment by changing the genotype which changes the phenotype of the organism. The phenotype trait is selected by the organisms and passed on to the generation to become permanent. Nature selects the fittest species to survive the environment.  

Since the organism reproduces either through asexual or sexual mode in which sexual mode increases the chances of natural selection as sexual mode involves the mixing of genes from different parents which increases the chances of "variation" in the genotype thus phenotype.

Thus, variation is the correct answer.