d
The amount of gas in the bubbles is fixed i.e. number of moles in both the bubbles initially = the number of final moles
${\mu _1} + {\mu _2} = \mu _1' + \mu _2'$
$\frac{{PV}}{{R(273)}} + \frac{{PV}}{{R(273)}} = \frac{{1.5\,PV}}{{R(273)}} + \frac{{1.5PV}}{{R(T)}}$
$\Rightarrow$ $\frac{2}{{273}} = \frac{{1.5}}{{273}} + \frac{{1.5}}{T}$
$\Rightarrow$ $T = 819\,K = 546^\circ C$