In test tube has snail a plant in b and both in c which has highest concentration of Carbon dioxide?

assuming the plant is alive:

B, the plant tube, would have the least CO2 (carbon dioxide) because the plant would use it in photosynthesis.

C, with both snail and plant, would have a medium or normal amount of CO2. The snail would use oxygen and produce CO2 as it breathed. The plant would be doing the opposite (using CO2, and producing oxygen)

Therefore, A, with only the snail, would obviously have the highest concentration of CO2. With no plant to use any, the concentration would build, and if it was a sealed test tube, the snail would eventually use up all the oxygen in the air and die of asphyxiation.