The chemical used for cloud seeding for artificial rain is

Cloud seeding is a weather-modification technique in which a chemical nucleating agent is dispersed into supercooled clouds (clouds whose water droplets are below 0 degree Celsius but have not yet frozen), so that ice crystals form around it; these crystals grow, gather more moisture, and eventually fall as precipitation.

Silver Iodide (AgI) has a hexagonal crystal lattice that closely matches the structure of natural ice, so water molecules readily arrange onto it and freeze even at temperatures only a few degrees below 0 degree Celsius. This efficient ice-nucleating property is why silver iodide, dispersed from aircraft or ground-based generators, is the standard chemical used to seed clouds and induce artificial rain.

• Silver Nitrate is valued for photographic film and silver-plating or mirroring, not for nucleating ice.

• Potassium Bromide is historically used as a sedative and in older photographic processes, not for cloud seeding.

• Potassium Nitrate is used in fertilizers and as an oxidizer in gunpowder or in food preservation, not to trigger ice-crystal formation in clouds.

So, among the substances listed, only the one with an ice-like crystal structure serves as an effective cloud-seeding agent.