The two main sources of genetic variation are: a) single-gene traits and polygenic traits. b) directional selection and disruptive selection. c) genotypes and phenotypes. d) gene shuffling and mutations.

d) gene shuffling and mutations.

Explanation:

During meiosis-I, the exchange of genetic segments between the homologous chromosomes occurs. This is called crossing over. The recombinant chromatids formed during crossing over have new combinations of alleles that were otherwise not present in the parental chromatids. Similarly, the random assortment of alleles of different genes towards the opposite poles of the cell during anaphase-I also creates new gene combinations. These events of gene shuffling during meiosis recombine the genetic material and add genetic variations in the progeny.  

When genetic material undergoes any random change in its sequence, the corresponding mRNA sequence is also altered. This results in the formation of faulty, nonfunctional, less functional or no protein from the mutated mRNA. Therefore, the sudden changes in DNA called mutations also add variations.