ANS:C - both(a) & (b)

In the case of constant underflow extraction operation, the solids are drained to the same extent at all solute concentrations. Constant underflow extraction refers to an extraction process where the ratio of the insoluble to the solution is kept constant. This means that the extraction process is designed in such a way that the concentration of solute in the solution leaving the extractor remains constant over time. As a result, the solids are drained to the same extent regardless of the solute concentration. Therefore, the correct statement is: solids are drained to the same extent at all solute concentrations.