What is the change in the phase if a wave is reflected from a denser medium

The correct option is π.

When a wave traveling through a rarer (less dense) medium encounters a boundary with a denser medium, the boundary behaves as a fixed or rigid support. This is because the particles of the denser medium have greater inertia and do not easily displace in response to the incident wave.

To satisfy the boundary condition at this interface, the net displacement of the particles at the boundary must remain zero at all times. For this to happen, the incident wave and the reflected wave must interfere destructively at the boundary. Consequently, the reflected wave is inverted relative to the incident wave.

An inversion of the wave's displacement (where a crest reflects as a trough, and a trough reflects as a crest) represents a phase change of 180°, which in radians is:
$\pi$
Therefore, when a wave is reflected from a denser medium, it undergoes a phase change of π radians.