Step 1: Locate the term in civil law.
A "plaint" is a foundational term under the Code of Civil Procedure (CPC), 1908, used to initiate a civil suit.
Step 2: Apply the definition.
A plaint is a written statement filed by the plaintiff (the party who files the suit) setting out the cause of action along with all necessary particulars to support the claim.
Step 3: Eliminate wrong options.
A plaint is not filed by a witness, advocate, or authority - it is exclusively the plaintiff's statement of claims.
\[ \boxed{\text{claims in which the plaintiff sets out his cause of action with all necessary particulars}} \]