Answer:
(D) Formation of mercury from mercuric oxide.
Step-by-step explanation:
The other three reactions involve the formation of a new substance from a chemical reaction between two or more substances. Specifically:
(A) Formation of calcium oxide from limestone involves thermal decomposition.
(B) Formation of aluminium from aluminium oxide involves reduction.
(C) Formation of sodium carbonate from sodium hydrogen carbonate involves thermal decomposition.
However, the reaction in option (D) involves the reduction of mercuric oxide to form mercury. This is different from the other reactions, which all involve the formation of a new substance from a chemical reaction.