Option A – Quick lime
CaCO₃ $\xrightarrow{\Delta}$ CaO + CO₂
The product 'X' is Quick lime (Calcium oxide, CaO).
Source: Chapter 1, Section 1.2.1
The thermal decomposition of calcium carbonate (limestone) gives calcium oxide (quick lime) and carbon dioxide. This is a standard decomposition reaction. The source passage (Ch. 1) identifies CaO as "Quick lime," which directly matches option A. Gypsum (CaSO₄·2H₂O) and Plaster of Paris are calcium sulphate compounds — unrelated to this reaction.