Final answer:
The characteristic always true for a single-displacement reaction is that one element replaces another element in a compound, like in the reaction 2 HCl(aq) + Zn(s) → ZnCl₂(aq) + H₂(g), where zinc replaces hydrogen. So, the option is A.
Step-by-step explanation:
The characteristic that is always true of a single-displacement reaction is that one element replaces another element in a compound. This type of reaction follows the general format A+BC → AC+B, where element A replaces element B in the compound BC, resulting in a new compound AC and a single element B.
A single-displacement reaction is also known as a single-replacement reaction, and these are often redox reactions, which involve a change in oxidation numbers because elements on the reactant side are in their free forms (oxidation number of zero) and then become part of a compound on the product side of the equation. An example of a single-displacement reaction is when zinc reacts with hydrochloric acid: 2 HCl(aq) + Zn(s) → ZnCl₂(aq) + H₂(g). Here, zinc replaces hydrogen in the compound.
Therefore, A is the option.