If John pressures Mary to sell their home by threatening legal action, the resulting sales contract may be voidable due to:

The correct answer is duress. Duress occurs when one party to a contract is forced to act against their will or judgment through threats or coercion. In this scenario, John is applying pressure on Mary by threatening legal action, which can create a situation where Mary feels obligated to agree to the sale of the home despite not wanting to do so. This coercive tactic undermines the principle of mutual consent that is foundational to contract law, meaning that Mary did not enter into the contract freely and willingly.

The other options, while they address different legal concepts, do not accurately describe this situation. Fraud involves deceitful practices that induce someone to enter into a contract, which is not indicated here. Legality of object pertains to whether the purpose of the agreement is lawful, and mental incapacity addresses situations where a person is not capable of understanding the nature of the agreement due to mental impairment. None of these apply to the coercive context presented in this scenario.