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:

Un mutagen es un agente que induce cambios en el ADN, como radiación o ciertos químicos. Las otras opciones no definen correctamente un mutagen.

Answer:

an agent that causes a mutation in DNA.