When the CAPM correctly prices risk the market portfolio is an efficient portfolio why?

The Capital Asset Pricing Model (CAPM) asserts that an efficient market portfolio consists of all risky assets, weighted by their market values, leading to an optimal risk-return trade-off. In this context, efficiency means that no other portfolio can offer a higher expected return for the same level of risk, or a lower risk for the same expected return. This efficiency arises from the assumption that all investors have access to the same information and make rational decisions, which drives the market to price assets in such a way that all portfoliOS on the efficient frontier reflect the risk associated with their expected returns. Thus, when CAPM accurately prices risk, it confirms that the market portfolio is indeed efficient.