Which of the following is something municipal bond investors cannot obtain?

Municipal bonds are known for their tax advantages, which make them attractive to investors. One of the notable features of municipal bonds is that interest income from these bonds is typically exempt from federal income tax. However, this exemption can be affected by certain factors, notably the Alternative Minimum Tax (AMT).

Investors in municipal bonds cannot obtain a federal tax exemption if the bonds they invest in are considered "private activity bonds" and are subject to the AMT. This condition can make the interest income from these particular bonds taxable under the AMT rules, hence influencing the overall tax benefit that an investor may expect from municipal bonds.

In the context of the other options, a federal tax exemption applies to many municipal bonds, while investing in qualified private activity bonds can still offer some tax exemptions, and state tax exemptions generally apply to municipal bonds based on the investor's state of residence. Thus, the correct choice highlights the specific limitations in regard to the AMT and the federal tax exemptions available on certain municipal bond investments.