In tube B, as there is only a snail, which as all animals needs aerobic respiration by mitochondria to obtain energy, it will eventually consume the oxygen and won't survive in closed conditions.
Tube C has a snail and a plant, which has the ability to make photosynthesis thanks to having chloroplasts, producing oxygen as well as glucose. The presence of the plant in the tube, will provide the snail enough oxygen to survive. Moreover, carbon dioxide from celular respiration of both the snail and the plant will serve as a substrate for photosynthesis. There is an equilibrium allowing all the species survival.
The presence of two snails in tube D, produces a too elevated oxygen consumption, as the plant cannot cope with such consumption, the snails will die.
An hypothesis for this predictions, is that animals need a certain oxygen concentration to survive, it is proportional to the population size and it can be obtained from plant photosynthesis.