Which one of the following reactions is different from the remaining three?

Let us analyze the types of chemical reactions represented by each option:

• Option A: $NaCl + AgNO_3 \rightarrow AgCl + NaNO_3$ is a double displacement reaction where ions are mutually exchanged between two compounds to form a precipitate ($AgCl$).
• Option B: $CaO + H_2O \rightarrow Ca(OH)_2$ is a combination reaction where two reactants combine to form a single product (slaked lime), as described in Activity 1.4 of the NCERT text.
• Option C: $KNO_3 + H_2SO_4 \rightarrow KHSO_4 + HNO_3$ is a double displacement reaction where ions are exchanged between the salt and the acid.
• Option D: $ZnCl_2 + H_2S \rightarrow ZnS + 2HCl$ is a double displacement reaction where zinc and hydrogen exchange their anions.

Since Option B is a combination reaction while all the other three options are double displacement reactions, Option B is different from the remaining three.