E. coli is the most well-understood bacterium and is an extremely important model organism in many fields of research, particularly molecular biology, genetics, and biochemistry. It is easy to grow under laboratory conditions, and research strains are very safe to work with. As with many bacteria, E. coli grows quickly, which allows many generations to be studied in a short time. Under ideal conditions, E. coli cells can double in number after only 20 minutes. The genetics of E. coli are well-understood and can be readily manipulated.
Therefore, the correct answer is option A.