ANS:C - both cases (a) and (b)

Facultative bacteria are capable of surviving and thriving in both aerobic (in the presence of oxygen) and anaerobic (in the absence of oxygen) conditions. They possess metabolic pathways that allow them to switch between aerobic respiration (using oxygen) and anaerobic respiration or fermentation (in the absence of oxygen). So, the correct option is: Both cases (a) and (b).