What defines a third-party defendant?

A third-party defendant is defined as a party brought into a lawsuit by an existing defendant who believes that this additional party is liable for some or all of the claims asserted in the original suit. This process is known as "impleader," where the original defendant seeks to bring another party into the litigation to ensure that complete justice can be achieved or to share responsibility for the claims being made.

In this context, option B accurately conveys this concept, highlighting the role of the third-party defendant in completing a controversy related to the case. This allows for any related claims to be resolved within the same legal proceedings rather than requiring separate lawsuits, thereby promoting judicial efficiency and fairness in resolving all issues at hand.

Other options do not align with the definition of a third-party defendant. For instance, someone who initiates an appeal does not have the role defined by a third-party defendant, nor does a party who files a civil suit or one who solely represents the plaintiff. Each of these roles pertains to different aspects of the legal process and litigation, making B the only choice that correctly identifies the nature of a third-party defendant.