The correct answer is D, They are backed by the full faith and credit of the U.S. government. Treasury securities (including Treasury bills, notes, and bonds) are considered among the safest investments because they are guaranteed by the U.S. government’s ability to tax and print money.

Step-by-step, this “full faith and credit” backing means that the government is legally obligated to pay interest and return principal on time, virtually eliminating credit (default) risk. This is why Treasury securities are often used as a benchmark for “risk-free” rates in financial markets.

Choice A is incorrect because Treasury securities are non-callable, meaning the government cannot redeem them prior to maturity. Choice B is incorrect because FDIC insurance applies to bank deposits, not securities. Choice C is incorrect because while Treasury securities are exempt from state and local taxes, they are still subject to federal income tax.

Thus, the defining characteristic of Treasury securities is their government backing, making them extremely low-risk investments and confirming that Answer D is correct.