Maximum work that could be secured by expanding the gas over a given pressure range is the __________ work.

The isentropic process is nothing but reversible adiabatic process since between the isothermal and adiabatic process the isothermal process has maximum area under P−V diagram the isothermal process will have maximum work done during expansion. Since the slope of p−v plot for adiabatic process is greater than isothermal process.