What is the importance of Moho discontinuity in identifying start of mantle below crust?

Correct answer: It is a clear seismic boundary between crust and mantle

The direct answer is C: the Moho is a clear seismic boundary between the crust and the mantle. Moho is short for Mohorovičić discontinuity. A discontinuity is a zone where the properties of Earth material change noticeably. At the Moho, seismic-wave velocity changes because the composition and density of the material below the crust differ from those in the crust. The reasoning is: observe earthquake waves; notice a sudden change in their speed or path; identify this boundary; infer that the mantle begins below it. Option C is correct. Option A is wrong because Moho is not a geographical edge of a river valley. Option B is wrong because it has no connection with monsoon limits. Option D is wrong because the outer core lies much deeper, below the mantle, and is not the surface marked by Moho. The depth of Moho varies under continents and oceans, but its meaning remains the crust–mantle boundary. Memory cue: “Moho means mantle starts below crust.”