The correct option is (a) 3:1
Best explanation: Mixing is achieved using an impeller mounted in the tank; for use with Newtonian fluids, the ratio of tank diameter to impeller diameter is normally about 3:1. The impeller is usually positioned overhead on a centrally-located stirrer shaft. For efficient mixing with a single impeller, the depth of liquid in the tank should be no more than 1.0-1.25 times the tank diameter.