What legal doctrine prevents the denial of a fact that was previously admitted to be true by action?

The concept that prevents the denial of a fact that was previously admitted to be true by action is known as estoppel. This legal doctrine ensures that a party cannot assert something contrary to an established fact that they have previously accepted.

In the context of insurance and other legal dealings, estoppel serves to promote fairness and consistency. For example, if a party has acted in reliance on a certain fact being true and has subsequently taken actions based on that belief, they cannot later deny the validity of that fact. This doctrine is critical as it protects individuals and parties from suffering detriment due to changes in another party's position regarding previously accepted facts or assertions.

While waiver refers to the relinquishment of a known right, and is also relevant in contractual and insurance contexts, it does not specifically relate to the denial of previously established facts. Concealment and misrepresentation pertain to withholding information or providing false information, respectively, and do not directly address the truth of a fact that has been acknowledged through previous actions or behavior. Thus, the choice of estoppel as the correct answer hinges on its foundational role in maintaining the integrity of legal assertions and the principles of fairness in negotiations and contracts.