A wave undergoes reflection from a rigid boundary. One of its characteristic parameters that changes is(a) frequency(b) phase(c) velocity(d) wavelength

When the wave undergoes reflection against a rigid body it results into change of shape so as to maintain the boundary condition at the wall. i.e. the particle at the boundary remains at rest.
Due to this there is change of phase.
Frequency, wavelength & velocity remains unchanged.

It can also be understood, consider wave as a ray which is about to incident on the rigid wall and since it is coming from upper part so after incident it will be reflect in lower part now replace ray by wave now wave is coming from upper part so after reflection it will move in lower.