Who are the individuals involved in a legal proceeding as either the plaintiff or defendant?

The correct answer identifies the individuals involved in a legal proceeding as "parties." In legal terminology, the term "parties" refers to the individuals or entities actively engaged in a lawsuit. This includes the plaintiff, who initiates the lawsuit, and the defendant, who is being accused or sued.

In contrast, the other terms provided represent different concepts in legal contexts. A vendor refers to someone who sells goods or services and is not inherently involved in a legal action unless related to a case in which they may serve as a party. A ward typically refers to a person, often a minor or someone unable to manage their own affairs, who is placed under the protection of a court; this person would not be classified as a party in a legal action unless they were involved in a specific case. The term next friend refers to an individual who represents another party in a legal proceeding, usually on behalf of someone who cannot represent themselves, such as a minor or incapacitated person. Although a next friend is involved in the legal process, they do not qualify as a party to the action themselves.

Overall, "parties" is the most appropriate term to describe those directly involved in legal proceedings as either plaintiffs or defendants.