Which of the following organisms completely lack a cell wall, are the smallest living cells known, and can survive without oxygen?

Mycoplasma is the smallest living cell, which is devoid of the cell wall. Without a cell wall, they are unaffected by many common antibiotics such as penicillin or other beta-lactam antibiotics that target cell wall synthesis. They can be parasitic or saprotrophic. Mycoplasma species are the smallest bacterial cells discovered so far which can survive without oxygen and are typically about 0.1 micrometers in diameter. Thus, the correct answer is option A.