Explanation

In modern portfolio theory, the efficient frontier represents the set of optimal portfolios that offer the highest expected return for a defined level of risk, or the lowest risk for a given level of expected return.

Based on standard portfolio theory:

• The efficient frontier includes all portfolios that lie on the upward-sloping portion of the minimum-variance frontier
• It excludes portfolios that are dominated by others (those with lower returns for the same risk or higher risk for the same return)
• The efficient frontier typically spans from the minimum variance portfolio to the maximum return portfolio

Given the options:

• Option C (X and Z) would typically represent the range from the minimum variance portfolio to the maximum return portfolio, which constitutes the efficient frontier

This is a fundamental concept in portfolio optimization where investors seek to maximize returns for a given level of risk by selecting portfolios along the efficient frontier.