The best student description of the reaction is:
This reaction is endothermic. A "cold pack" where ammonium nitrate and water react can pull heat from the surroundings to the system so the pack feels cold.
This response correctly identifies the reaction as endothermic, explaining that the reaction absorbs heat from the surroundings, resulting in the cold sensation associated with the cold pack. The details provided accurately describe the process and the science behind it.