"when barium hydroxide is mixed with ammonium chloride, ice crystals form on the outside of the container" is the correct answer to describe the endothermic process.
The third one down: "When barium hydroxide is mixed with ammonium chloride, ice crystals form on the outside of the container."
This is because an endothermic reaction [process] takes in more energy than it releases. Therefore because a decrease in energy leads to a decrease in temperature, the ice crystals forming show that it must be a endothermic reaction [process].
