(c) Dissolution of ammonium chloride in water
Endothermic reactions absorb energy from the surroundings. Dissolving NH₄Cl in water absorbs heat, making the solution cold — a classic endothermic process.
The key definition: endothermic = energy absorbed. Options (a), (b), and (d) all release heat (exothermic). Dissolution of NH₄Cl is the standard textbook example of an endothermic process because it cools the surroundings. Examiners expect you to recall this directly from Chapter 1 examples.
Source: Chapter 1, Chemical Reactions and Equations