Final answer:
The correct answer is a), where hydrochloric acid reacts with bromine to form hydrogen bromide and chlorine. This is a displacement reaction and is distinct from a composition reaction demonstrated by the reaction of hydrogen and chlorine gases to form hydrochloric acid.
Step-by-step explanation:
The reaction 2HCl + Br2 → 2HBr + Cl2 describes the process where hydrochloric acid (HCl) reacts with bromine (Br2) to form hydrogen bromide (HBr) and chlorine gas (Cl2). In context, the correct answer to the student's question is: a) Hydrochloric acid reacts with bromine to form hydrogen bromide and chlorine. This reaction is not a composition reaction; it is a type of displacement reaction, specifically a halogen displacement reaction, as it involves the exchange of halogens. A composition reaction, also known as a synthesis reaction, is one in which two or more substances combine to form a single new product, as in reaction (a) H2 + Cl2 → 2HCl, where hydrogen and chlorine gases react to form hydrochloric acid.