Why do heat expand when it is added to most solids?

The faster molecular motion leads to greater distance between the molecules

The atomic theory of matter states that matter is made up of tiny, rapidly moving particles. These particles move more quickly when warmer, because temperature is a measure of average kinetic energy of the particles. Warmer molecules therefore move further away from each other, with enough energy to separate from each other more often and for greater distances. The individual molecules do not get bigger, by conservation of mass, eliminating answer (A). The molecules do not develop greater repelling electric forces, eliminating answer (D). Occasionally, molecules form a more rigid structure when becoming colder and freezing (such as water) --but this gives rise to the exceptions to heat expansion, so it is not relevant here, eliminating answer (D). Therefore, the answer is (B).