Answer:
C. long-term; zero coupon bond.
Explanation:
If the investor is worried about declining interest rates on his present Investment, the best option to get is long term zero coupon bond.
They are long term in nature. Zero coupon bonds pay the full face value on maturity. It does not have periodic payments to the beneficiary. At maturity the investor will receive the par value.
Zero coupon bonds do not pay interest so are not subject to interest fluctuations. It operates on a discount and at maturity the full value is guaranteed.