The pleading which starts a civil case is the filing of the:

The filing of a complaint is the initial step in commencing a civil case. A complaint is a formal legal document that outlines the plaintiff's claims against the defendant, detailing the facts and legal reasons that support the plaintiff's request for relief. It serves to notify the defendant of the allegations made against them and initiates the legal proceedings.

While a summons is also an important document, it is used to notify the defendant that they are being sued and to compel their appearance in court, but it does not initiate the case on its own. A demurrer, on the other hand, is a response to a complaint that challenges the legal sufficiency of the claims made within the complaint, and a subpoena is a document used to order someone to appear in court or produce evidence, which comes into play at a different stage in the legal process. Thus, the complaint is the correct answer as it is the foundational pleading that begins a civil lawsuit.