b
Since the gas expands adiabatically (i.e., no change in enthalpy) so the heat is totally converted into work.

For the gas \(C_V=20\,J/K.\) Thus, \(20\,J\)  of heat is required for \(1^o\) change in temperature of the gas.

Heat change involved during the process 

i.e., work done \(3\,kJ=3000\,J\)

Change in temperature \(=\frac {3000}{20}\,K=150\,K\)

Initial temperature \(=300\,K\)

Since, the gas expands so the temperature decreases and thus final temperature is 

\(300-150=150\,K\)