A market maker typically trades as principal. Acting as principal means the firm buys and sells securities for its own account and stands ready to provide liquidity by quoting bid and ask prices. The market maker buys at the bid and sells at the ask, earning the spread as compensation for committing capital and assuming inventory risk. Choice C is correct. An agent or broker acts on behalf of a customer and generally earns a commission rather than trading from inventory. Although a firm can act in different capacities in different transactions, the defining role of a market maker is principal trading. "Fiduciary" describes a legal standard of care, often associated with investment advisers, not the trading capacity of a market maker. The SIE outline identifies traders and market makers as market participants and also requires candidates to understand trade capacity, including principal versus agency. This question tests that distinction directly: brokers arrange trades for others, while dealers and market makers trade for their own accounts. Reference: Section 1.1.4 Market Participants and Their Roles; Section 3.1.1 Orders and Strategies.