Final answer:
The correct answer to the question is A. More; non-mutant to mutant. The Ames test uses a histidine auxotroph strain of bacteria to determine the mutagenicity of substances by counting revertant colonies able to grow without histidine after exposure to a potential mutagen.
Step-by-step explanation:
During the Ames test, if a substance is a mutagen, there will be more colonies on the plates in which the cells had been exposed to the mutagen, because the mutagen converts some of the cells from non-mutant to mutant. Therefore, the correct answer is A. More; non-mutant to mutant.
The Ames test is a method that uses bacteria to assess the mutagenic potential of chemical compounds. A strain of Salmonella typhimurium that cannot make its own histidine due to a mutation, known as a histidine auxotroph, is exposed to a potential mutagen. If the substance is a mutagen, these bacteria may gain back the ability to synthesize histidine, allowing them to grow on a medium lacking histidine. These revertant colonies can then be counted and compared to control samples where no mutagen was present. An increased number of revertant colonies suggest that the substance being tested has mutagenic properties.