a) lattice waves, free electrons
b) lattice waves, free protons
c) free protons, free electrons
d) lattice waves, longitudinal waves

Answer: a
Explanation: Thermal energy is transported through the molecules due to lattice waves and free electrons. In pure metals, the conduction of heat is based on the flow of free electrons and the effect of lattice waves is considered as negligible. In alloys and nonmetallic solids, since they are only few free electrons, conduction of heat is due to lattice vibrations in between the molecules.