In a single displacement (replacement) reaction, what occurs?

In a single displacement (or replacement) reaction, one reactant is replaced by another, which is why this answer is correct. This type of reaction typically involves an element and a compound, where the free element displaces one of the elements in the compound, resulting in a new element and a new compound. For example, in a reaction between zinc and hydrochloric acid, zinc displaces hydrogen to form zinc chloride and hydrogen gas.

The other choices do not accurately describe what happens in a single displacement reaction. The first option implies that two compounds react to form a single element, which is characteristic of a different type of reaction. The third option refers to the combination of elements to form a compound, which describes a synthesis reaction. Lastly, the fourth option describes a decomposition reaction, where a single compound breaks down into simpler substances, which is not the focus of a single displacement reaction.