Concept: This question tests the understanding of various types of chemical reactions and their characteristics.

Why (B) is correct:

• A. Combination reaction: A reaction in which two or more reactants combine to form a single product. The reaction $2Mg(s)+O_2(g)\to 2MgO(s)$ fits this definition as magnesium and oxygen combine to form magnesium oxide. Thus, A matches with S.
• B. Decomposition reaction: A reaction in which a single compound breaks down into two or more simpler substances. The reaction $CaC{O}_3(s)\stackrel{\text{Heat}}{\to }CaO(s)+C{O}_2(g)$ is a classic example where calcium carbonate decomposes into calcium oxide and carbon dioxide. Thus, B matches with Q.
• C. Displacement reaction: A reaction in which a more reactive element displaces a less reactive element from its compound. The reaction $Fe(s)+CuS{O}_4(aq)\to FeS{O}_4(aq)+Cu(s)$ shows iron displacing copper from copper sulfate solution. Thus, C matches with P.
• D. Double displacement reaction: A reaction in which two compounds react by an exchange of ions to form two new compounds. The reaction $AgN{O}_3(aq)+NaCl(aq)\to AgCl(s)+NaN{O}_3(aq)$ involves the exchange of ions (Ag+ with Na+, and NO3- with Cl-) to form silver chloride precipitate and sodium nitrate. Thus, D matches with R.

Option Analysis:

• A) A→S; B→P; C→R; D→Q: Incorrect, as B, C, and D are mismatched.
• B) A→S; B→Q; C→P; D→R: Correct, all matches are accurate based on the definitions of reaction types.
• C) A→S; B→R; C→P; D→Q: Incorrect, as B, D are mismatched.
• D) A→S; B→Q; C→R; D→P: Incorrect, as C and D are mismatched.

Correct Answer: (B)