The correct answer is: D. money market mutual funds.
Money market mutual funds are a type of mutual fund that invests in short-term, low-risk securities, such as Treasury bills, commercial paper, and negotiable certificates of deposit. They are considered to be a safe investment, and they are often used as a place to park cash until it is needed for another purpose.
Treasury bills are short-term debt obligations issued by the U.S. government. They are considered to be one of the safest investments available, and they are often used as a benchmark for other investments.
Commercial paper is a type of short-term debt issued by corporations. It is considered to be a riskier investment than Treasury bills, but it also offers a higher yield.
Negotiable certificates of deposit are a type of short-term debt issued by banks. They are similar to CDs, but they can be traded between investors before they mature.
All of these investments are considered to be low-risk, and they are often used by businesses as a place to park cash until it is needed for another purpose. However, money market mutual funds are the most liquid of these investments, and they are also the most tax-efficient.