In an adiabatic change, the pressure and temperature of a mono atomic gas are related as  PαTC, where c equals

in adiabatic process relation between Pressure P,Temperature T is P1-γTγ=constant ⇒TγPγ-1=Constant ⇒Pγ-1αTγ ⇒PαTγγ−1     given PαTC  on comparing the above two equations  C=γγ−1for monoatomic gas specific heat ratio γ=53  substitute γ in above equation of C  C=5353−1 ⇒C=52