An investor who wants tax-free investment income would choose a (an) _______ fund.

An investor seeking tax-free investment income would choose a municipal bond fund because the interest earned from municipal bonds is generally exempt from federal income tax, and oftentimes from state and local taxes as well, depending on the investor's location and the specific bond. This makes municipal bonds particularly appealing to individuals in higher tax brackets who are looking to maximize their after-tax returns.

Municipal bonds are typically issued by state and local governments to finance public projects such as schools, highways, and infrastructure improvements. By investing in these bonds, an investor can support their community while also benefiting from tax advantages. In contrast, other options like government bonds and treasury bills usually generate interest that is subject to federal income tax, and while some of their income may be exempt from state and local taxes, it does not provide the same level of tax exemption as municipal bonds. Minimum security does not refer to a widely recognized investment vehicle and could create confusion, adding to why municipal bonds are the best choice for tax-free investment income.