Final answer:
The defense of mutual mistake requires that both parties are mistaken about a fundamental fact, and this mistake must materially affect the contract. (option d) However, the element 'the party who is not seeking to assert the defense assumed the risk of mistake' is not a requirement for establishing mutual mistake.
Step-by-step explanation:
To establish the defense of mutual mistake in contract law, the following elements are typically required: both parties must be mistaken about a fact at the time the contract is made; the mistake must relate to a basic assumption on which the contract is based; and the mistake must have a material impact on the bargain. However, the party who is not seeking to assert the defense assumed the risk of mistake is not a required element for the defense of mutual mistake. In contrast, if one of the parties has assumed the risk of a mistake then the defense of mutual mistake may not be available since the assumption of risk means that the party has accepted the possibility of the existence of a mistake.