ANS:C - both (a) and (b).

Evaporation by thermocompression involves compressing vapor from the evaporator to increase its pressure and temperature, which allows for further evaporation without the need for additional external heat. This process results in the saving of steam, as it allows for the reuse of vapor generated in the evaporation process. Additionally, while thermocompression itself does not directly result in the realization of multiple effect economy in a single effect, it can be used in conjunction with multiple effect evaporators to further enhance steam economy and efficiency. Therefore, the correct option is: saving of steam.