b
$P V=n R T=\frac{w}{M} R T$
Hence, $P=\frac{\rho}{M} R T$
Initially, $P V=\frac{w}{M} R T_{1}-(1)$
After heating, $P=\frac{\rho}{M} R T_{2}$
From equations $( 1 )$ and $( 2).$
The temperature to which the gas was heated is
$T_{2}=\frac{w T_{1}}{\rho V}=\frac{12 \times 280}{6 \times 10^{-4} \times 4 \times 10^{3}}=$$1400$