Ans. (D). causing mutations with chemicals or radiation.
Genetic variations can be defined as variation between different individuals and populations. Variations occur due to presence of different nucleotide sequences in different organisms.
Scientists can produce new genetic variations that are not present in nature by introducing mutations in organisms. Mutations are permanent changes in genetic material of an organism, due to alterations in nucleotide sequence. Mutations are caused by mutagens, which include chemical agents (such as nitrous acid) or physical agents (such as radiation).
Thus, the correct answer is option (D).