The half life for the decomposition of gaseous compound A is 240s...

${\left(t\dfrac{1}{2}\right)}_{500torr} = 240sec = 4min$

${\left(t\dfrac{1}{2}\right)}_{250torr} = 4min .$

$t\dfrac{1}{2} ∝ {a}^{1 - n}$

$\dfrac{{\left({t}_{\dfrac{1}{2}}\right)}_{1}}{{\left(t\dfrac{1}{2}\right)}_{2}} = {\left(\dfrac{{a}_{1}}{{a}_{2}}\right)}^{1 - n}1 = {2}^{0} = {\left(\dfrac{250}{500}\right)}^{1 - n}n = 1$

As $t\dfrac{1}{2}$ is independent of initial pressure. Hence, order is $1$st order.