(a) Both (A) and (R) are true and (R) is the correct explanation of (A).
Quicklime (CaO) reacts vigorously with water releasing a large amount of heat, making it an exothermic reaction. (R) directly explains why (A) is true.
Source: Chapter 1, Section 1.2.1 Combination Reaction
The textbook explicitly states: "Calcium oxide reacts vigorously with water to produce slaked lime releasing a large amount of heat" and defines exothermic reactions as those in which heat is released along with products. Since (R) gives the exact reason for (A), option (a) is correct — not (b). Students often confuse (a) and (b); choose (b) only when the reason, though true, explains something else.