The Markowitz efficient frontier represents the set of portfolios that offer the highest expected return for a given level of risk, or the lowest risk for a given level of return. It is the upper portion of the minimum-variance frontier, not the entire frontier.

• Option A is incorrect because the efficient frontier does not lie above and to the left of the minimum-variance frontier; it is the upper part of it.
• Option B is incorrect because the entire collection of minimum-variance portfolios for all possible returns is called the minimum-variance frontier, not the efficient frontier.
• Option C is correct because the efficient frontier includes all portfolios of risky assets that rational, risk-averse investors would choose, as these portfolios provide the optimal risk-return trade-off.