Correct choice is (a) Stirred tank batch reactors
To elaborate: The above diagram represents a reactor wherein the products are taken out in batches. This type of reactors contains a tank with stirrer for proper mixing between enzyme and substrate and giving products. This kind of reactor is stirred tank batch reactors (STRs). In STRs, enzymes and substrates have identical residence times within the reactor. The operating costs of batch reactors are higher than for continuous processes due to the necessity for the reactors to be emptied and refilled both regularly and often.