While dissolving 80.0 g of ammonium nitrate in 50.0 mL of water at room temperature, you observe that the temperature of the solution
decreases by 5.0°C and the vessel feels cold to the touch. You
realize that the formation of the solution must be an endothermic
process!

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.

dynamic equilibrium is the answer.