Structural materials that are intended for service at very high temperatures are usually strengthened by precipitation hardening, using precipitates that involve substitutional solute species. Examples include the Fe-based and Ni-based 'superalloys' used in jet engines, which are strengthened by precipitates of intermetallic compounds such as Ni3Ti. Why choose precipitation hardening? Why should the precipitates employ substitutional solutes?

answer is: c. the experiment shows that the red substance experienced a chemical change.

this is chemical change (chemical reaction), because two new substances are formed (metalig liquid and gas), the atoms are rearranged and the reaction is followed by an energy change.

this is example of chemical decomposition.

chemical decomposition (analysis) is the separation of a single chemical compound into its two or more elemental parts or to simpler compounds.  

chemical decomposition is opposite of chemical synthesis.