Achemical reaction is a process in which

A chemical reaction is a process in which one or more substances, called reagents, are transformed into products

Explanation:

The most common definition of a chemical reaction is that it is a process in which one or more substances, called reagents, are transformed into another or other substances with different properties, called products.  During a chemical reaction, the bonds between the atoms that constitute the reagents are broken. Then, the atoms are reorganized in another way, forming new bonds and giving rise to one or more substances different from the initial ones.

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.