Which of the following portfolio is risk free?

Select one:

a. A portfolio consisting of $100,000 worth of US TIPS.

b. A portfolio consisting of $100,000 worth of long-term U.S. T-bonds.

c. A portfolio consisting of $100,000 worth of U.S. corporate junk bonds.

d. A portfolio consisting of $100,000 worth of 30-day T-bills. Every 30 days the T-bill matures, and you reinvest the principal in a new batch of T-bills.

e. A portfolio consisting of $100,000 worth of AAA-rated U.S. corporate bonds.