Amutagen is

a) a living thing that has undergone a mutation.
b) an agent that causes a mutation in dna.
c) a mutation that has affected one gene.
d) a chemical that can poison living cells.

The correct answer is B) An agent that causes a mutation in the DNA.

Mutagen is a chemical or a physical agent that brings change in the genetic material of an individual and therefore increases the chances of mutation(permanent change in the genetic material).

Therefore, it is an agent that causes mutation in the DNA.

Example of mutagen- Ionizing radiation (like X- rays) , UV radiations.

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