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, etc.) that induces changes (mutations) in the DNA sequence. A living thing with a mutation is a mutant. A mutation affecting one gene is just a single - gene mutation. A chemical that poisons cells is a toxin (not necessarily causing a mutation).

Answer:

an agent that causes a mutation in DNA.