Which is the most common mechanism of genetic variation in the population of a sexually reproducing organism?

The most common mechanism of genetic variation in the population of a sexually reproducing organism is recombination. Recombination occurs during meiosis. The main reason for recombination is crossing over which is the exchange of exchange of genetic material between non-sister chromatids of two homologous chromosomes.

Transduction is the transfer of genetic material from one bacterium to another with the help of bacteriophage.
Chromosomal aberration is a type of mutation in which a portion of chromosome is duplicated, deleted or exchanged between non-homologous chromosomes. This is not a common mechanism of genetic variation in the population of a sexually reproducing organism.
Genetic drift is the change in allelic frequency occurring by chance. Sometimes the change in allele frequency is so different in the new sample of population that they become a different species. This might lead to evolution.