Step 1: Definition of market.
In economics and sociology, a market is not only a place of exchange but also a structured institution where buyers and sellers interact.
Step 2: Explanation.
It is considered an institution because it organizes trade, regulates interactions, and provides norms for transactions, rather than simply being a random group of people.
Step 3: Conclusion.
Therefore, the correct answer is (A) An institution.
Final Answer: \[ \boxed{\text{An institution}} \]