Question

at the mantle - core boundary p - waves
reflect into the core
diffract towards the crust
diffract into the core
refract into the core and back into the mantle
rewatch

Explanation:

When P - waves (primary waves) reach the mantle - core boundary, due to the change in material properties (the core has different physical characteristics than the mantle), P - waves refract. Refraction occurs when a wave passes from one medium to another and changes its direction. P - waves do not just reflect into the core (reflection would imply bouncing back in a more specular way without the wave entering the core in a refracted sense). Diffraction is more about bending around obstacles rather than the behavior at a major boundary like the mantle - core boundary. And the description of refracting into the core and back into the mantle is incorrect as it's a one - way refraction at the boundary (from mantle to core) based on the wave's path and the properties of the two layers.

Answer:

refract into the core