Under what condition can supplemental jurisdiction be denied according to § 1367(b)?

Supplemental jurisdiction is a mechanism that allows federal courts to hear additional claims that are related to the main claim already before them. Under § 1367(b), there are specific conditions under which supplemental jurisdiction can be denied, particularly concerning claims involving new parties.

The correct answer reflects a situation where the addition of new plaintiffs can lead to the denial of supplemental jurisdiction. In essence, if the main claim is originally based in federal jurisdiction, and the additional claim seeks to introduce new plaintiffs who do not meet the diverse citizenship requirements necessary for federal jurisdiction, then the court may decline to exercise supplemental jurisdiction over those claims. This provision is intended to preserve the principles of diversity jurisdiction and ensure that cases do not inadvertently bring in parties that would disrupt the diversity requirements outlined in § 1332.

Thus, when considering the implications of adding new plaintiffs, if their involvement changes the jurisdictional landscape—for instance, introducing claims that would prevent the federal court from having diversity jurisdiction—the court is compelled to deny supplemental jurisdiction. This is a safeguard against undermining the established jurisdictional standards.