Answer: Have the highest rates of return for a given level of risk.
Step-by-step explanation:
Efficient Portfolios offer the highest rates of return for a given level of risk or the lowest risk for a given return. This means that they always maximise returns for a given level of risk which makes them very attractive to the point that they are labeled 'The Optimal Portfolio'.
It is argued at at this level, including any other Investment vehicle or rather diversifying the portfolio further cannot be done unless more risk is accepted. This is the point where the portfolio can get no less riskier for the return it offers.