Question

a mutagen is
a living thing that has undergone a mutation.
an agent that causes a mutation in dna.
a mutation that has affected one gene.
any chemical that can poison living cells.

Explanation:

A mutagen is defined as an agent (which can be chemical, physical like radiation, or biological) that causes changes in the DNA sequence, leading to mutations.

Answer:

B. an agent that causes a mutation in DNA.