The correct answer is: A. aggressive investors.
Aggressive investors are willing to take on more risk in order to achieve higher returns. Portfolios lying on the upper right portion of the efficient frontier are riskier than portfolios lying on the lower left portion of the efficient frontier, but they also have the potential to generate higher returns.
Conservative investors, on the other hand, are less willing to take on risk. They prefer to invest in portfolios that are less risky, even if this means that they will not earn as high of a return.
Risk-averse investors are somewhere in between aggressive and conservative investors. They are willing to take on some risk, but they are not as willing to take on as much risk as aggressive investors.
Defensive investors are the most risk-averse type of investor. They prefer to invest in portfolios that are very low-risk, even if this means that they will not earn as high of a return.