Explanation

(b) The efficiency of cycle is
$\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \eta = 1 - \frac{T_{2}}{T_{1}}$
For adiabatic process
$\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ TV^{\gamma - 1} = constant$
For diatomic gas $\gamma = \frac{7}{5}$
$\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ T_{1}V_{1}^{\gamma - 1\ } = T_{2}V_{2}^{\gamma - 1}$
${\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ T}_{1} = T_{2}\left( \frac{V_{2}}{V_{1}} \right)^{\gamma - 1\ }$
$T_{1} = T_{2}(32)^{\frac{7}{5} - 1} = T_{2}\left( 2^{5} \right)^{2/5} = T_{2} \times 4$
${\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ T}_{1} = 4T_{2}$
$\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \eta = \left( 1 - \frac{1}{4} \right) = \frac{3}{4} = 0.75$