Answer: d. Zero coupon, long maturity
Explanation:
It is generally held that when interest rates decrease in the market, the price of bonds will increase because people will seek bonds as they offer a steady rate of return.
A longer maturity bond will enable you to take advantage of this decrease in interest rates over a longer period because you get to discount the bond at a lower rate over a longer period so it is better.
A zero coupon long maturity bond is the best because when it is discounted at this lower rate, it will bring back a higher price than the rest of the bonds
