Answer: C (sodium). The thermal stability of a metal carbonate increases as the metal becomes more reactive. The carbonates of the most reactive metals (Group I, such as sodium) are very stable and are not decomposed by the heat of a Bunsen burner.
Carbonates of less reactive metals decompose more easily on heating to give the oxide and carbon dioxide — for example copper(II) carbonate and zinc carbonate decompose readily, and calcium carbonate needs strong heating.
Of calcium, copper, sodium and zinc, sodium is the most reactive, so sodium carbonate is the most thermally stable.
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