An ideal gas is compressed to half its initial volume by means of several processes. Which of the process results in the maximum work done on the gas?

Since work done is equal to the area under the P-V curve between the initial and final volume. As seen from a P-V diagram (which should be included in a complete answer, but cannot be created here in text format), this area is maximum for the adiabatic process, less for the isothermal process, and least for the isobaric process. Therefore, Wadiabatic > Wisothermal > Wisobaric. The isochoric process involves no change in volume, so no work is done (Wisochoric = 0).