(a)
$\because\ \mathrm{\Delta}T = \frac{W}{nR}\therefore Q = nC_{p}(\mathrm{\Delta}T) = nC_{p}\frac{W}{nR} = \frac{C_{p}W}{R}$
$C_{p} = \frac{QR}{W} = \frac{500 \times 2}{142.8} = 7$
$C_{p} = 7$indicates that the gas is diatomic. Thus, it should be $O_{2}$