Answer: I'm gonna say the answer is Mutualism.
Evolution can be defined as the process of the change occurring in a population over a period of time through genetic variation. This genetic variation and the process of natural selection help a species to evolve by selecting the traits that helps it to better adapt to the environment. These genetic variations are inherited over time as the species continue to evolve.
Hence, the correct answer is 'change in population through genetic variation over time'.
Actually, all four of them are correct. Bacteria are responsible for all of them and they are very important.
