This problem includes concept of isothermal adiabatic irreversible expansion.
Process is adiabatic because of the use of thermal insolution therefore, q = 0
Internal energy can be written as
The change in internal energy of an ideal gas depends only on temperature and change in internal energy (ΔU) = 0 therefore, ΔT=0 hence, process is isothermal and
(d) p2Vγ2=p1Vγ1 is incorrect, it is valid for adiabatic reversible process.
Hence, only (a), (b) and (c) are correct choices.