No two people are genetically identical, except for identical twins. The main source of genetic variation among humans is

a. new mutations that occurred in the preceding generation.

b. genetic drift.

c. the reshuffling of alleles in sexual reproduction.

d. environmental effects.

c. the reshuffling of alleles in sexual reproduction.

Explanation:

During gametogenesis, meiosis occurs in reproductive cells. As a result, haploid gametes are produced. During meiosis, a process called 'crossing over' occurs in the pachytene stage of prophase I of meiosis. During crossing the chromosome segments are exchanged between non-sister chromatids of homologous chromosomes. Exchange of chromosome segment creates a new combination of genes or genetic recombination and produces hybrids. Thus, genetic variation occurs in all sexually reproducing organisms due to crossing over.