Artificial way of causing rainfall to reduce air pollution makes use of

1. A

   silver iodide and potassium iodide

   Silver iodide and potassium iodide. AgI is the standard ice-nucleating agent; KI appears with it in many seeding mixes.

2. B

   silver nitrate and potassium iodide

   Silver nitrate and potassium iodide. AgNO3 is not the usual nucleant.

3. C

   silver iodide and potassium nitrate

   Silver iodide and potassium nitrate. The iodide is right; KNO3 is the wrong partner.

4. D

   silver nitrate and potassium chloride

   Silver nitrate and potassium chloride. Both names miss the iodide nucleant.

Summary. Official key is (a) silver iodide and potassium iodide — the classic cloud-seeding pair. Nitrate/chloride options swap in the wrong silver or potassium salt.