Right answer is (a) True
For explanation I would say: Large airlift reactors with capacities of thousands of cubic metres have been constructed. Tall internal-loop airlifts built underground are known as deep- shaft reactors; very high hydrostatic pressure at the bottom of these vessels considerably improves gas-liquid mass-transfer. The height of airlift reactors is typically about 10 times the diameter; for deep-shaft systems the height-to-diameter ratio may be increased up to 100.