Step 1: Picture the setup.
Two identical gas systems sit side by side. The thin wall between them is pulled out so the two combine into one bigger system.
Step 2: What happens to volume.
Volume is extensive, meaning it adds up. Two parts of volume $V$ each give a total of $V+V = 2V$.
Step 3: What happens to temperature.
Both parts were already at the same temperature $T$. Mixing two equally warm gases does not change how hot they are, so temperature stays $T$.
Step 4: What happens to pressure.
Each side already pushed with pressure $P$, and the gas is the same on both sides. After removing the wall nothing changes the push, so pressure stays $P$.
Step 5: What happens to density.
Mass doubles and volume doubles together, so $\dfrac{2m}{2V} = \dfrac{m}{V} = d$. Density stays $d$.
Step 6: Collect the new values.
So the combined system has $2V, T, P, d$, which is the last option.
\[ \boxed{2V,\ T,\ P,\ d} \]