Answer:
The correct option is D. Crossing over and independent assortment are the causes of genetic variation.
Step-by-step explanation:
In the process of meiosis, crossing over is a phenomenon through which exchange of genetic segments takes place among the homologous chromosomes.
The process of independent assortment describes how different genes assort differently during meiosis.
Both the process of crossing over and independent assortment are the basis of the genetic variations seen in the world.