Final answer:
Option 3, A+B → AB, represents a synthesis reaction where two reactants combine to form a single new substance, consistent with the definition of a synthesis or combination reaction.
Step-by-step explanation:
Synthesis Reaction Identification
The correct answer for which reaction represents a synthesis reaction is option 3: A+B → AB. A synthesis, or combination, reaction is one in which two or more reactants combine to form a single product. This type of reaction is characterized by the general formula A+B → AB, where A and B are the reactants that come together to form a single compound AB.
Comparing this to the other options given, option 1 represents a decomposition reaction, where a compound breaks down into its constituent parts. Option 2 and 4 represent exchange reactions (also known as double replacement or double displacement reactions), where parts of the reacting compounds are exchanged to form new compounds.