If a plaintiff sues a defendant and that defendant impleads a third party, can the plaintiff now sue the third party?

The correct answer is indeed that the plaintiff can sue the third party under the proper conditions. In the context of procedural law, once a defendant has impleaded a third party into the case, the plaintiff can assert claims against that third party. This is allowed as long as the claims meet the requirements for jurisdiction and can arise from the same transaction or occurrence related to the original claim.

This mechanism promotes judicial efficiency, ensuring that all related disputes are resolved in one action, rather than forcing the plaintiff to file a separate lawsuit against the third party. It is important that any claims against the third party adhere to jurisdictional limits and relevant rules about joinder, such as Rule 14 of the Federal Rules of Civil Procedure, which governs third-party practice.

The other options either misunderstand the procedural rules or impose unnecessary conditions that do not reflect the nature of impleader and the plaintiff's rights in the litigation process.