Only a _______ may earn a commission and only a _______ may sue to collect a commission.

In the context of real estate and agency law, the correct statement is that only a broker may earn a commission and only a broker may sue to collect a commission. This is rooted in the nature of the broker's role and legal standing in transactions involving real estate or similar domains.

Brokers are licensed professionals who typically have more extensive training and legal authority than agents. They can operate independently and represent clients in transactions, whereas agents usually work under a broker. Since only licensed brokers hold the legal standing to negotiate and enter into contracts that involve the collection of commissions, they are the only parties authorized to claim such fees in a legal context.

Therefore, the answer accurately reflects the regulatory framework that governs real estate transactions and the earning of commissions, confirming that the role of brokers is distinct and carries specific responsibilities and rights that agents do not possess.