Answer: Option D). a mutation.
Explanation:
Mutation is the change in the genetic composition of the cell. This change in a gene can be due to
- spontaneous change in nitrogenous base
- error during replication i.e incorrect copying of DNA template strand
- damage to DNA by ionizing radiation such as X-rays
- damage to DNA by heavy metals such as cadmium, lead etc
Thus, mutation is the answer.
