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.

• Option 1: A living thing that has undergone a mutation is called a mutant, not a mutagen.
• Option 2: This is the correct definition of a mutagen as it refers to the agent causing the DNA mutation.
• Option 3: A mutation affecting one gene is just a description of a type of mutation, not what a mutagen is.
• Option 4: A chemical that can poison living cells is a toxin (in a general sense if it's a chemical), not necessarily a mutagen (a mutagen specifically causes DNA mutations).

Answer:

an agent that causes a mutation in DNA.