A stock exchange is a marketplace where securities such as stocks, bonds, and other financial instruments are bought and sold. It provides a platform for companies to raise capital by offering shares to investors. The primary functions of a stock exchange include:

• Facilitating the exchange of securities between buyers and sellers, thereby providing liquidity to investors.
• Ensuring fair trading practices by regulating and monitoring transactions.
• Providing a mechanism for price discovery where the price of a security is determined based on supply and demand.
• Offering a secure and regulated environment to conduct transactions.

Given the options provided:

• Commodities are bought and sold at wholesale price: This activity is typically associated with commodity markets, not stock exchanges.
• Commodities are bought and sold at retail price: Similarly, retail pricing of commodities is not within the domain of stock exchanges.
• Securities are bought and sold: This is the correct activity that occurs at a stock exchange.
• None of these: Incorrect, as securities are indeed bought and sold at a stock exchange.

Therefore, the correct answer is: Securities are bought and sold.