The longer the chromosomes of an organism, the more genetic variability it is supposed to get from which of the following processes?

The correct option is C Crossing over
Option C is the correct answer.
Genetic variability in an organism arises due to exchange of genetic materials between non-sister chromatids of homologous chromosomes. This occurs during meiosis. In prophase I, the homologous chromosomes pair and exchange their genetic materials by undergoing a process called crossing over. Crossing over leads to recombination which gives rise to genetic variability in offspring.
The extent of crossing over and recombination depends on the points where crossing over occurs. In a pair of homologous chromosomes which are longer, the chances of crossing over in a longer segment of the chromatids gives rise to more variability than in a pair of homologous chromosomes which are short.
Independent assortment states that during the formation of gametes, the segregation of one pair alleles of a gene into the gametes is independent of the segregation of alleles of the other gene. It is responsible for the genetic variability to some extent but it is independent of the length of the chromosome. It is dependent on the placement of genes, whether they are located on the same chromosomes or different chromosomes.
Linkage is the phenomenon in which 2 genes have a tendency to remain associated together because of their placement on a single gene. Linkage usually acts as a hindrance to genetic variability.
Mutation can occur due to any change in the genetic material of the organism. It can either happen in the chromosomes or in the DNA. Mutation is also responsible for genetic variability but is random and independent of length of chromosome.