The correct answer is: A. efficient portfolio.
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 risk premium is the additional return that an investor expects to earn for taking on additional risk. The standard deviation of a portfolio is a measure of its volatility.
To calculate the expected return rate on an efficient portfolio, you would add the riskless rate to the risk premium and multiply that by the standard deviation of the portfolio.
The other options are incorrect because they do not describe an efficient portfolio. A non-efficient portfolio is a portfolio that does not maximize expected return for a given level of risk, or does not minimize risk for a given level of expected return. An attributable portfolio is a portfolio that is attributed to a specific investor or group of investors. A non-attributable portfolio is a portfolio that is not attributed to a specific investor or group of investors.