In the case of a longitudinal wave, such as a sound wave, the particles of the medium vibrate parallel to the direction of wave propagation. This creates regions of compression and rarefaction in the medium.

In the region of rarefaction, the particles are spread apart, resulting in a decrease in density. This is because the particles move away from each other as the wave passes through.

Let's break down the answer into the following points:

1. Volume momentarily increases:
- In the region of rarefaction, the particles of the medium are spread apart, leading to an increase in the volume of that region.
- This is because the particles move away from each other, creating more space between them.
- As a result, the volume of the medium in the region of rarefaction momentarily increases.

2. Density momentarily decreases:
- Density is defined as mass per unit volume.
- In the region of rarefaction, the particles are spread apart, which means there are fewer particles in a given volume.
- As a result, the mass per unit volume decreases, leading to a decrease in density in the region of rarefaction.

3. Pressure momentarily decreases:
- Pressure is defined as force per unit area.
- In the region of rarefaction, the particles are spread apart, resulting in less frequent collisions between particles and with the walls of the container (if present).
- As a result, the force exerted by the particles on a given area decreases, leading to a decrease in pressure.

Conclusion:
In summary, in the region of rarefaction of a longitudinal wave, the volume momentarily increases because the particles move away from each other. This leads to a decrease in density because there are fewer particles in a given volume. Additionally, the pressure momentarily decreases because there are fewer collisions between particles and with the walls of the container. Therefore, all the options mentioned in 'D' (volume increases, density decreases, and pressure decreases) are correct.

Explanation:
In the region of rarefaction of a longitudinal wave, the particles of the medium are spread out or have a lower density than in the equilibrium position. This results in several changes in the wave parameters.
The changes in the region of rarefaction are:
- The volume momentarily increases: The particles in the rarefaction region move away from each other, causing an increase in the volume of the medium. This is because the particles are spread out and have more space between them.
- The density momentarily decreases: As the particles move apart, the density of the medium in the rarefaction region decreases. This is because there are fewer particles per unit volume in the rarefaction region compared to the equilibrium position.
- The pressure momentarily decreases: The decrease in density in the rarefaction region leads to a decrease in pressure. As the particles move apart, there are fewer collisions between them, resulting in a lower pressure compared to the equilibrium position.
Therefore, all of the above changes occur in the region of rarefaction of a longitudinal wave.