Which of the following activities requires a real estate broker's license in Illinois?

The activity that requires a real estate broker's license in Illinois is representing anyone in a real estate transaction. This is because engaging in activities that involve the representation of clients in real estate deals, such as buying, selling, or leasing property, falls under the purview of licensed activities. A real estate broker is legally authorized to act on behalf of clients, which includes negotiating contracts, facilitating transactions, and ensuring compliance with real estate laws.

Each of the other options involves specific activities that might not require a full broker's license. For instance, managing a property for an owner can sometimes be done under limited circumstances without a license if it's a specific type of management or if the manager is an employee of the owner. Helping someone find a rental may not require a license if it's done informally or as a service without the intent of compensation or representation. Advising sellers on property pricing can fall under consulting or advisory services that don't necessarily require a broker’s license unless the advisor is also involved in the transaction itself.

In the broader context of real estate regulations in Illinois, the necessity for a broker's license serves to protect consumers by ensuring that individuals representing others in real estate transactions are knowledgeable and adhere to professional standards.