The most direct cause of variation in the offspring of sexually reproducing organisms is the mixing of genetic material from two parents during fertilization.

Here's why:

* Independent assortment: During meiosis, the process that produces gametes (sperm and egg), chromosomes from each parent are shuffled and separated randomly. This means that each gamete receives a unique combination of chromosomes.

* Crossing over: During meiosis, homologous chromosomes (one from each parent) exchange genetic material, leading to even more variation in the resulting gametes.

* Random fertilization: Any sperm can fertilize any egg, further increasing the potential for variation.

These processes ensure that each offspring receives a unique combination of genes from its parents, resulting in a diverse range of traits within a population.