To increase the efficiency of a pulley system operating at 50% efficiency, the best option is:
by reducing the friction on the pulleys.
Lowering friction allows more of the input energy to be effectively used to lift the load, thus increasing overall efficiency. Other options, such as adding more pulleys or having a second person pulling, might not necessarily increase efficiency and could even complicate the system without guaranteeing improved performance. Reducing the amount of weight being lifted doesn't improve efficiency; it merely reduces the load the system has to handle.