What is supplemental jurisdiction not available for in diversity cases?

Supplemental jurisdiction permits federal courts to hear additional claims that are closely related to the original jurisdiction claims in order to promote judicial economy and avoid duplicate litigation. However, in diversity cases, supplemental jurisdiction cannot be used to hear claims that would defeat the complete diversity required for federal jurisdiction.

In the case of claims against persons made parties under the impleader rules, this situation often arises when a defendant brings a third party into the case potentially altering the jurisdictional landscape. If a plaintiff attempts to add a defendant from the same state as themselves (destroying complete diversity), supplemental jurisdiction would not apply. This reflects a fundamental principle that diversity jurisdiction is based on the complete diversity of citizenship of the parties involved in the litigation.

The remaining scenarios typically do not present the same jurisdictional issues. A plaintiff's claim against a defendant in the same jurisdiction or against a third-party defendant does not necessarily destroy the complete diversity required if certain conditions are met. Similarly, a defendant’s claim against a third-party plaintiff does not affect the existing jurisdiction if it relates back to the same transaction or occurrence as the original claim.

Therefore, the correct answer highlights the limitations of supplemental jurisdiction in preserving the complete diversity requisite in diversity cases.