When a longitudinal wave is incident at the boundary of denser medium, then ____________.
A. Compression reflects as a compression
B. Compression reflects as a rarefaction
C. Rarefaction reflects as a compression
D. Longitudinal wave reflects transverse wave

Hint On reflection at the boundary of a denser medium, a change of phase by $180^\circ $ takes place for a longitudinal wave. In other words, there is a phase difference of $180^\circ $ between the incident wave and the reflected wave.

 Complete step-by-step solution:
Let us first discuss about what is compression and rarefaction.
In the motion of longitudinal wave, a compression is a formed when the particles comes very close to each other whereas a rarefaction is formed when the particles goes far apart from each other.
In other words, we can also say that the region where the medium is compressed is known as a compression and the region where the medium is spread out is known as a rarefaction.
Suppose that longitudinal waves in air are incident perpendicularly on a rigid wall (See the diagram). When a compression hits the wall, it exerts a force on the wall and the wall also exerts an equal and opposite force on the particles in compression and hence pushes the compression in the opposite direction.

Thus, there is a phase difference of $180^\circ $ between the incident wave and the reflected wave.
Therefore, a compression which is travelling towards the wall is reflected as a compression in the direction opposite to the wall and a rarefaction incident on the wall is reflected as a rarefaction.
Hence, option A is correct.

Note: When a compression is formed in a medium, the density and pressure of the medium increases whereas when a rarefaction is formed in the same medium, density and pressure of the medium decreases.
Therefore, compression is also known as the region of high density and pressure whereas rarefaction is known as the region of low density and pressure.