You place ammonium nitrate crystals in water and stirred. As you do so, the container becomes cold to the touch. This an example of

Exothermic reaction

An endothermic reaction

A physical reaction

A heat pack reaction

The process of flask becoming cold is due to endothermic reaction.

Answer: Option B

Explanation:

So two kinds of heat transfer can be possible in any chemical reaction. If the sample is considered as system and the sample container is considered as the surrounding, then heat transfer can occur between them.

If the heat is transferred from the surrounding to the system, then it is an endothermic reaction. And in those cases, the sample holder will be becoming colder. This is because the heat from the surrounding that is the container will be utilized to complete the reaction.

While when there is transfer of heat from the system to surrounding, it will be exothermic reaction and the beaker will be getting hot in this process. So in the present case, the container is becoming cold because of occurrence of endothermic process.