The substance used as a smoke screen in warfare is

ANS:B - SiCl₄

Silicon tetrachloride (SiCl4) is a chemical compound consisting of one silicon atom bonded to four chlorine atoms. It is a colorless, volatile liquid that fumes in air. SiCl4 is primarily used in the production of silicon for semiconductor devices and in the synthesis of various silicon-containing compounds. In terms of military applications, silicon tetrachloride is not commonly used for generating smoke screens or concealment. It is less suitable for this purpose compared to substances like phosphorus pentoxide (P2O5), which react vigorously with atmospheric moisture to produce dense smoke.