**Explanation:**

In a longitudinal wave, particles of the medium vibrate parallel to the direction of wave propagation. This means that the particles move back and forth in the same direction as the wave is traveling. This motion creates regions of compression, where the particles are close together, and regions of rarefaction, where the particles are spread apart.

In these regions of compression and rarefaction, several physical quantities can change. However, the one physical quantity that does not change is the mass of the particles.

**Pressure:**
In a longitudinal wave, the particles in the regions of compression are pushed closer together, increasing the pressure. Conversely, in the regions of rarefaction, the particles are spread apart, decreasing the pressure. Therefore, the pressure is not constant in these regions.

**Density:**
Density is defined as mass per unit volume. In a longitudinal wave, the particles in the regions of compression are closer together, resulting in a higher density. Conversely, in the regions of rarefaction, the particles are spread apart, resulting in a lower density. Therefore, the density is not constant in these regions.

**Volume:**
In a longitudinal wave, the particles in the regions of compression are pushed closer together, resulting in a smaller volume. Conversely, in the regions of rarefaction, the particles are spread apart, resulting in a larger volume. Therefore, the volume is not constant in these regions.

**Mass:**
The mass of an object is the amount of matter it contains and is a fundamental property of matter. In a longitudinal wave, the particles simply oscillate back and forth about their equilibrium positions, but the amount of matter in each particle remains constant. Therefore, the mass of the particles does not change in these regions.

Thus, in a longitudinal wave, the physical quantity that does not change in the regions of compression or rarefaction is the mass of the particles.