Choice "C" is correct. A surety may raise his or her own contract defenses to limit his or her liability; thus, the surety's own incapacity is a defense to the surety promise.
Choice "a" is incorrect. The debtor's discharge in bankruptcy is not available as a defense to the surety. A debtor's possibility of going bankrupt is one of the main reasons that creditors require sureties.
Choice "d" is incorrect. A surety generally cannot raise the debtor's personal defenses against the creditor.
Choice "b" is incorrect. A surety cannot raise the debtor's incapacity as a defense against the creditor. A debtor's infancy or mental incapacity is one of the main reasons why a creditor might require a surety.