Which pair of variables for a gas is inversely proportional to each other, assuming other conditions are constant?

The question asks which pair of variables for a gas is inversely proportional to each other, assuming other conditions are constant. We use the ideal gas law, $PV = nRT$, where $R$ is the ideal gas constant.

• A) n, P: If $V$ and $T$ are constant, then $P = \frac{nRT}{V}$. Since $RT/V$ is constant, $P \propto n$. This means $P$ and $n$ are directly proportional.
• B) P, T: If $V$ and $n$ are constant, then $P = \frac{nRT}{V}$. Since $nR/V$ is constant, $P \propto T$. This means $P$ and $T$ are directly proportional (Gay-Lussac's Law).
• C) V, T: If $P$ and $n$ are constant, then $V = \frac{nRT}{P}$. Since $nR/P$ is constant, $V \propto T$. This means $V$ and $T$ are directly proportional (Charles's Law).
• D) P, V: If $n$ and $T$ are constant, then $PV = nRT$. Since $nRT$ is constant, $PV = \text{constant}$. This means $P \propto \frac{1}{V}$, so $P$ and $V$ are inversely proportional (Boyle's Law).

Therefore, the pair of variables that are inversely proportional to each other is P and V.

The correct option is (D).
$\boxed{\text{D}}$