Diversifying investments in a portfolio can help to reduce portfolio risk. Diversification can be achieved by all of the following EXCEPT:

Diversification in a portfolio refers to the strategy of spreading investments across various financial instruments, sectors, or other categories to mitigate risk. While buying high-quality assets can enhance the overall performance and stability of a portfolio, it does not inherently contribute to diversification.

Investing solely in the highest quality assets tends to concentrate holdings in only certain types of securities or issuers, thus failing to achieve the broadening of the investment base that diversification seeks. This concentration increases the portfolio's exposure to specific risk factors associated with a limited segment of assets or sectors, rather than spreading risk across different perspectives that diversification aims to achieve.

On the other hand, limiting investments in particular business sectors, restricting investments from specific issuers, and varying the credit quality of investments all serve to diversify a portfolio. These strategies ensure that the portfolio is not overly reliant on one source of risk, thereby reducing the potential negative impact of adverse events on specific investments.