What does increase in P-wave velocity while entering mantle from crust indicate?

Correct answer: Entry into more rigid and denser material

The direct answer is B: entry into more rigid and denser material. P waves are compressional seismic waves. Their speed depends on the material’s elasticity, rigidity, density and state. At the Mohorovičić discontinuity, or Moho, seismic properties change sharply as waves pass from crust into mantle. Mantle rock is generally more rigid and denser than crustal material, so P-wave velocity increases. A is wrong because increasing air would usually reduce rigidity and would not explain the mantle boundary. C is unrelated: ocean-water expansion does not cause this seismic discontinuity. D concerns atmospheric pressure, whereas P waves travel through the solid Earth. The reasoning is: crust-to-mantle boundary → changed, more rigid dense rock → faster P waves. Exam cue: a sudden seismic-speed change marks a discontinuity such as the Moho.