Answer
The process A to C.
Work Step by Step
Isobaric process implies constant pressure.
$PV=NKT$, where $P$ is the pressure, $V$ is the volume, $N$ is the number of particles of the gas, $K$ is Boltzmann's constant, and $T$ is the temperature.
$N$ and $K$ are constants.
$P$ is constant implies $V\propto T$.
Since $V$ has doubled from $V_o$ to $2V_o$, $T$ will double from $T_o$ to $2T_o$. Thus A to C.
