What doctrine bars a legal action due to unreasonable delay by the affected party?

The correct answer, which is the doctrine of laches, is crucial in preventing a party from asserting a claim if there has been an unreasonable delay in pursuing that claim, and that delay has prejudiced the other party. Laches is grounded in the principle that equity aids the vigilant, not those who sleep on their rights. For instance, if a plaintiff waits an excessively long time to file a lawsuit and their delay negatively impacts the defendant's ability to respond or mounts a defense (perhaps because evidence has been lost, or witnesses are no longer available), the court may refuse to hear the case altogether.

In contrast, negligence pertains to a breach of duty leading to harm, res judicata prevents the same issue from being litigated multiple times after a final judgment, and estoppel prevents a party from claiming something contradicting what they previously established as true. Each of these doctrines serves different legal principles and functions within the judicial system, highlighting why laches is specifically appropriate for barring actions due to an unreasonable delay.