The correct answer is (a) Mandatory.
The principle of res judicata is a legal principle that states that a final judgment on the merits of a case by a court of competent jurisdiction is conclusive of the rights of the parties and prevents them from relitigating the same issues in a subsequent action. This principle is mandatory, meaning that courts are required to apply it, even if the parties do not want it to apply.
The principle of res judicata is based on the idea that finality is important in the legal system. Once a case has been decided, the parties should be able to rely on the judgment and not have to worry about the possibility of the case being relitigated. This principle also helps to promote judicial efficiency, as it prevents courts from having to hear the same case multiple times.
There are a few exceptions to the principle of res judicata. For example, a court may not apply the principle if the judgment was obtained by fraud or mistake, or if there has been a change in circumstances since the judgment was issued.
The principle of res judicata is a fundamental principle of the legal system. It helps to ensure finality in litigation and promotes judicial efficiency.