Which best explains how the collisions of materials in space contribute to the formation of layers in protoplanets? The materials undergo decay when they collide, which results in the heating and subsequent melting and rising of materials.
The collisions release heat, which results in the heating and subsequent melting, sinking, and rising of materials.
The materials undergo decay when they collide, which results in the cooling and subsequent hardening and sinking of materials.
The collisions absorb heat, which results in the cooling and subsequent hardening and rising of materials.

The best explanation is;

The materials undergo decay when they collide, which results in the heating and subsequent melting and rising of materials

Explanation: A protoplanet is an embryo formed in a protoplanetary disc which has passed through a melting phase that enables the formation layered interior

In protoplanets the effects of partial melting of the components due to heating produced by radioactive decay and  pressures from forces of gravity there is segregation of the melt and igneous composition such that the heavier melted metal can sink and be over laid by the lighter igneous rocks

Therefore, the best explanation is that the materials undergo decay when they collide, which results in the heating and subsequent melting and segregation by the sinking of the heavier melted materials and rising of the lighter igneous materials.