A scientist thinks that a certain chemical is a mutagen. She exposes plant cells to a large amount of this chemical in the laboratory. Which statement best provides evidence that the substance is a mutagen?
The cells die within hours of being exposed to the chemical.
The cells grow more quickly than those that were not exposed to the chemical.
The cells change after being exposed to the chemical, and this change is passed to the next generation of cells.
The cells continue to divide at the same rate as before they were exposed to the chemical.

The cells change after being exposed to the chemical, and this change is passed to the next generation of cells.

Explanation:

A mutation is any change, whether big or small, that occurs to the nucleotide sequence of a gene in a cell as caused by mistake during replication or induced by a mutation causing substance. These mutation-causing substances are called MUTAGEN.

A MUTAGEN is any substance that causes genetic mutation in an organism's cell. These changes in the nucleotide sequence can affect one or more phenotypic characteristics of the affected organism, which is also passed to offsprings because it is a genetic change.

Hence, a plant cell exposed to this mutagen will be induced to mutate.

Hence, the cells changing after being exposed to the chemical, which is also passed to the next generation of cells best provides explanation that the chemical is a MUTAGEN.

the answer is:

carbon dioxide