Step 1: Understanding the Zeer.
The Zeer is a traditional cooling device that relies on the principle of evaporative cooling. The wet sand around the inner pot facilitates the evaporation of water, which absorbs heat and cools the inner pot.
Step 2: Conclusion.
Thus, the cooling in this product occurs due to evaporative cooling, where water evaporation from the sand absorbs heat.
\[
\boxed{\text{The correct answer is (b) Evaporative cooling.}}
\]