$PV = \large\frac{w}{m}RT$
$P = \large\frac{d}{m}RT$
$P_1 = \large\frac{d_1}{m_1}RT_1$
$P_2 = \large\frac{d_2}{m_2}RT_2$
$\therefore \large\frac{P_1}{P_2} = \large\frac{d_1}{d_2}\times\large\frac{T_1}{T_2}$
$\Rightarrow \large\frac{1}{2}\times\large\frac{2}{1}$
$\Rightarrow 1$
Hence answer is (a)