All of the following make a contract voidable if present at its formation, except:

B is incorrect because duress makes a contract voidable. When someone is forced into an agreement through threats or pressure, they can choose to void it rather than being bound by it automatically.

C is incorrect because undue influence makes a contract voidable. This occurs when someone uses their position of power or trust to improperly persuade another party, allowing the victim to cancel the contract.

D is incorrect because an illegal purpose makes a contract void, not voidable. Contracts violating public policy or the law are automatically unenforceable from the beginning.