a
\(\Delta U = \mu \,{c_v}\Delta T\) ==> \({U_2} - {U_1} = \mu \,{c_v}({T_2} - {T_1})\)

\({T_1} = 0\) so \({U_1} = 0\) 

\({T_2} = T\) and \({U_2} = U\)

\(U = \mu \,\,{c_v}T = \mu \,T \times {c_v}\)\( = \frac{{PV}}{R} \times \frac{R}{{\gamma - 1}} = \frac{{PV}}{{\gamma - 1}}\)

[\(PV = \mu \,RT\)                      \(\therefore\) \(\mu T = \frac{{PV}}{R}\) and \({c_v} = \frac{R}{{\gamma - 1}}\)]