d
$150 \mathrm{J}$ of heat has been added to the gas
From the first law of thermodynamics
$\Delta \mathrm{U}=\mathrm{Q}+\mathrm{W}$
For isothermal process, $\Delta U=0$
therefore, $Q=-w$
Given $\mathrm{W}=-150$
Therefore, $Q=+150$
When $Q$ is positive, the heat is added to the gas.