Thermal stability increases with decreasing charge density of the cation Steps:

• Identify cations: Ca²⁺ (Group 2), Cu²⁺ and Zn²⁺ (small d-block), Pb²⁺ (large but post-transition).
• Thermal stability of MCO₃ depends on cation's polarizing power; higher charge density (charge/size) polarizes CO₃²⁻ more, easing decomposition to MO + CO₂.
• Compare sizes: Cu²⁺ (73 pm) and Zn²⁺ (74 pm) smallest, highest density; Ca²⁺ (100 pm) moderate; Pb²⁺ (119 pm) largest but weaker M-O bonds.
• Thus, CaCO₃ requires highest temperature (~840°C) for decomposition.

Why A is correct:

• Ca²⁺ has lower charge density than d-block ions, polarizing CO₃²⁻ less per Fajans' rules, stabilizing the carbonate.

Why the others are wrong:

• B: Cu²⁺ small size, high charge density causes easy polarization and decomposition (~200°C).
• C: Pb²⁺ large but inert pair effect weakens Pb-O bonds, allowing decomposition (~350°C).
• D: Zn²⁺ small d-block ion with high charge density leads to low stability (~300°C).

Final answer: A