The correct answer is (c). Smoke screens are used in warfare for the purpose of concealment and camouflage. They generally consist of fine particles of titanium oxide dispersed in air. These particles scatter light, making it difficult to see through the smoke. Smoke screens can also be used to create a false target or to obscure the movement of troops.
(a) Sodium chloride is table salt. It is not a good material for smoke screens because it is not dense enough to scatter light effectively.
(b) Silver iodide is a white powder that is used in photography and in some types of smoke screens. It is denser than sodium chloride, but it is still not as effective as titanium oxide.
(d) Magnesium oxide is a white powder that is used in some types of smoke screens. It is denser than silver iodide, but it is still not as effective as titanium oxide.
Titanium oxide is the best material for smoke screens because it is dense enough to scatter light effectively and it is not toxic.