Step 1 - Understanding endo and exothermic reactions
A chemical reaction may absorb or liberate heat. We can measure the heat involved in a reaction, which is called enthalpy.
If a reaction absorbs heat, it is called endothermic, which comes from the greek preffix endo, meaning inside, i.e., heat flows to the reaction.
On the other hand, when a reaction liberates heat, it is called exothermic, which also comes from the greek exo, meaning outside, i.e., heat flows out of the reaction.
Step 2 - Answering the exercise
When a chemical reaction uses up energy, it is absorbing energy. Therefore, it is an endothermic reaction.
Answer: 3. endothermic
