Low default-risk security issued by financially secure firms is classified as

The correct answer is A. U.S treasury bills.

U.S. Treasury bills are considered to be the safest investment in the world. They are backed by the full faith and credit of the U.S. government, which means that they are virtually risk-free. Treasury bills are also very liquid, meaning that they can be easily bought and sold.

Commercial paper is a short-term debt instrument issued by large corporations. It is considered to be a relatively safe investment, but it is not as safe as Treasury bills. Commercial paper is not backed by the full faith and credit of the government, so there is a small risk of default.

A certificate of deposit (CD) is a time deposit at a bank or credit union. CDs are considered to be a safe investment, but they are not as liquid as Treasury bills or commercial paper.