A firm acting as agent makes money by:

When a firm acts as an agent, it does not trade securities for its own account but facilitates transactions between buyers and sellers. The firm earns revenue by charging a commission to its clients for executing these transactions.

Profiting from the spread (A): This is typical for firms acting as principal, not as agent.

Providing advice (B): Advice is part of advisory services, not agency transactions.

Trading against its own order book (D): This describes proprietary trading, not agency.