The correct answer is: C. redeemable at refund
A redemption option which protects investors against rise in interest rate is considered as redeemable at refund. This means that the bond issuer can call the bond back at any time and pay the investor the face value of the bond, plus any accrued interest. This option is valuable to investors because it protects them from the risk of interest rates rising and the value of their bonds falling.
A bond that is redeemable at deferred means that the bond can only be redeemed after a certain period of time, usually 5 or 10 years. This option is less valuable to investors because it does not protect them from the risk of interest rates rising in the short term.
A bond that is redeemable at par means that the bond can be redeemed at its face value, regardless of the market value of the bond. This option is not very valuable to investors because it does not protect them from the risk of interest rates falling and the value of their bonds rising.
A bond that is redeemable at finding means that the bond can be redeemed at any time, but the issuer must pay a premium to the investor. This option is the least valuable to investors because it does not protect them from the risk of interest rates rising or falling.