Choice "D" is correct. Any variation on an uncompensated surety's risk releases the surety.
Choice "c" is incorrect. If the release includes a reservation of rights against the surety, the surety is not discharged.
Choice "a" is incorrect. The fact that the principal debtor is bankrupt is not a defense to the surety.
Choice "b" is incorrect. The fact that the principal debtor was incompetent at the time the contract was made is not a defense available to the surety.
