The correct answer is: A. set of attainable portfolios.
An efficient portfolio is a portfolio that maximizes expected return for a given level of risk, or minimizes risk for a given level of expected return. The set of attainable portfolios is the set of all portfolios that can be constructed from a given set of assets. The set of attainable portfolios is bounded by the efficient frontier, which is the set of all portfolios that have the highest expected return for a given level of risk.
The set of unattainable portfolios is the set of all portfolios that cannot be constructed from a given set of assets. The set of unattainable portfolios is located outside of the efficient frontier.
The set of attributable portfolios is the set of all portfolios that are consistent with an investor’s risk tolerance and return objectives. The set of attributable portfolios is located within the efficient frontier.
Therefore, the first step in determining an efficient portfolio is to consider the set of attainable portfolios.