Answer: a. At the bottom of the tube, where oxygen is completely depleted
Explanation: Organisms that can only grow in the absence of oxygen are known as obligate anaerobes. These organisms don't require oxygen to undergo cellular respiration, instead, oxygen is actually toxic to them and hinders their growth. (since oxygen can be very reactive and interfere with vital metabolic processes)
On the other hand, organisms that require oxygen to grow are known as obligate aerobes. These organisms need oxygen to carry out the subsequent steps of cellular respiration. Without it, they won't produce enough ATP to survive and carry out cellular functions, leading to their death.
In a thioglycolate tube, the area near the top of the liquid has the highest oxygen concentration, and that oxygen concentration decreases until the very bottom of the tube. Therefore, obligate anaerobes such as Clostridium would be found near the bottom of the tube, as that location would have the lowest oxygen levels out of anywhere else in the tube. In the meantime, obligate aerobes would be found near the top of the tube to best take advantage of the high levels of oxygen availiable for cellular respiration.