If a scientist thinks that a certain chemicals is a mutagen and expose plant cells to a large amount of such chemical in the lab, the statement that best provides evidence that the substance is a mutagen would be that the cells change after being exposed to the chemical, and this change is passed to the next generation of cells.
A mutagen is generally described as a chemical that is capable of causing mutation in cells. Mutation in itself refers to heritable changes to the genome of a cell or an organism.
Hence, if a chemical is suspected to be a mutagen, such chemical must be able to cause mutation in the cells if they are exposed to it in large amount. The chemical should force some changes to the cell and the changes should be inheritable by the next generation of cells.
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