(B) The reaction of a reactive metal with dilute acid
For example: $\text{Fe} + \text{H}_2\text{SO}_4 \rightarrow \text{FeSO}_4 + \text{H}_2\uparrow$ — the metal displaces hydrogen from the acid.
A displacement reaction occurs when a more reactive element displaces a less reactive element from its compound. In option (B), a reactive metal displaces hydrogen from the acid. Option (A) is a combination/combustion reaction, (C) is decomposition, and (D) is also a combination reaction. Examiners expect you to identify the correct type and may award the mark only for (B).