Which among the following cause the variation ?

All of these .

All of the options mentioned—sexual reproduction, cross-pollination, and xenogamy—can contribute to genetic variation within a population.

Sexual reproduction, as mentioned before, involves the fusion of gametes from two parent organisms, leading to offspring with unique combinations of genes and promoting genetic diversity.

Cross-pollination, which is a type of sexual reproduction, refers to the transfer of pollen between different flowers, either within the same plant or between different plants. This process facilitates the exchange of genetic material, allowing for genetic variation within plant populations.

Xenogamy is a specific type of cross-pollination that involves the transfer of pollen between different species. This process can introduce genetic variation not only within populations but also across species boundaries.

Therefore, all three processes—sexual reproduction, cross-pollination, and xenogamy—can contribute to genetic variation in different ways and to different extents.