a
The ideal yas equation is given by -
$P V=n R T$
Since $T$ is constant in this pestion,
So, $P V=$ constant.
$\therefore \quad P_{1} V_{1}=P_{2} V_{2}$
Here, $V_{1}=1200 ml , V_{1}=1200 ml , P_{1}=70 cm$
$P_{2}=120 cm \text {. }$
Substitute, $70 \times 1200=120 \times V_{2}$
$\therefore v_{2}=\frac{70 \times 1200}{120}=700 ml \text {. }$
Correct choice - option - $A$