What must be defined to ensure best execution for a client?

To ensure best execution for a client, it is essential to define "the optimal outcome in terms of market conditions over time." Best execution involves executing trades at the most favorable terms for the client, which naturally requires a comprehensive understanding of current and historical market conditions. This optimal outcome reflects the ability to balance factors such as price, speed of execution, and liquidity, as well as the overall market environment.

By evaluating the market conditions over time, advisers can determine how different factors impact execution quality and help ensure that trades are executed in a way that reflects the best possible outcome for the client. Defining this optimal outcome aids in fulfilling fiduciary responsibilities and aligning the adviser’s actions with the client’s best interests.

The other options do not adequately address the complete framework needed for ensuring best execution. Prior commission rates alone do not inform whether an execution is truly optimal in the current market context. Focusing solely on expected returns of a client's portfolio lacks the necessary focus on the execution process and transaction specifics required to achieve best execution. Finally, while volume of trades might present some insights, it does not encapsulate the broader factors influencing execution quality. Hence, the emphasis on defining the optimal outcome contextualizes the execution within the framework of comprehensive market evaluation.