In this eight-year experiment, 12 populations of E. coli, each begun from a single cell, were grown in low-glucose conditions for 20,000 generations. Each culture was introduced to fresh growth medium every 24 hours. Occasionally, samples were removed from the populations, and their fitness in low-glucose conditions was tested against that of members sampled from the ancestral (common ancestor) E. coli population.
The cells in the 12 cell lines grown in low-glucose conditions showed the effects of which of the following processes?
A) gene flow and genetic drift
B) natural selection and mutation
C) natural selection and gene flow
D) conjugation and transformation

Answer is option "B"

Explanation:

Natural selection is the process in which the organism better suited for its environment will thrive and produce more offspring. This process plays a central role in evolutionary theory. As the demands of the environment act upon specific traits in a population, changes occur to improve the survival of the organism. The demands of the environment either benefit or work against specific traits that already exist within the population. A mutation is a change in your DNA, more specifically, a mutation that allows your DNA to create a protein that functions differently than it otherwise would. In humans, scientists have uncovered a recent mutation in the receptor proteins of the cell membrane. People possessing this mutation (though few in number) have shown a resistance to HIV. This is due to the inability of the virus to bind correctly to the host cell. Another example of beneficial mutation in humans is our rich, color vision.Hence, the right answer is option B "natural selection and mutation"