Step 1: In a civil legal proceeding, the parties have specific names. The person who initiates the case by bringing a complaint is called the plaintiff.
Step 2: Therefore a plaintiff is the one who files a case in a civil court, making option (a) correct.
Step 3: The opposing party who defends against the claim is the defendant, so option (b) describing a defender is wrong. The person who delivers the verdict is the judge, so option (c) is wrong. Since a valid definition exists, option (d) None is also incorrect.
Step 4: Hence the plaintiff is the person who files the case in a civil court.