The correct answer is option 4: Silver iodide. Silver iodide is commonly used to induce artificial rain.

Let`s analyze the other options to understand why they are not used for this purpose:

Option 1: Ammonium chloride. Ammonium chloride is a salt that is primarily used as a fertilizer and in various industrial processes. It does not have the properties necessary to induce rain.

Option 2: Calcium carbonate. Calcium carbonate is a commonly found compound in nature, such as in limestone and chalk. It is primarily used in building materials and the production of paper. It does not have the ability to induce rain.

Option 3: Potassium nitrate. Potassium nitrate is a type of salt that is mainly used as a fertilizer and in the production of gunpowder. It does not have the properties required to induce rain.

Therefore, out of all the options, silver iodide is the most suitable compound for inducing artificial rain. When silver iodide is dispersed in the atmosphere, it acts as a nucleating agent, promoting the formation of ice crystals in clouds. These ice crystals then grow into raindrops, resulting in precipitation.