What kinds of investments are allowed in a qualified retirement plan, and what kinds are forbidden?
Generally, the permissible investments for qualified plans include publicly-traded securities, real estate, mutual funds and money market funds. Some plans even allow investment of options. To be sure, always refer to the plan document which will provide a description of the investment options under the plan and a description of any restrictions.
For more insight, see the 401(k) and Qualified Plans tutorial.
This question was answered by Denise Appleby