In the context of Use Case modeling, an actor is fundamentally defined as an entity that is external to the system being analyzed or developed. An actor can be a person, another system, or an organization that interacts with the system to achieve a goal. The purpose of a use case is to describe the system's behavior from the perspective of these external entities. The system boundary clearly separates the internal components of the system from the external actors that use its functionality. Therefore, an actor always resides outside the system.

A. Inside: Entities that are part of the system's internal workings are considered components, classes, or subsystems, not actors.

C. Both Inside and Outside: This is incorrect. The definition of an actor is strictly external to the system boundary to model interactions with it.

D. None of the above: This is incorrect because option B provides the accurate and standard definition of an actor in use case analysis.

1. Object Management Group (OMG). (2017). Unified Modeling Language (UML) Specification, Version 2.5.1. Section 18.1.2.1, "Actors," p. 639. The specification states, "An Actor models a type of role played by an entity that interacts with the subjects (e.g., by exchanging signals and data), but which is external to the subjects." (The "subject" refers to the system).

2. Fowler, M. (2003). UML Distilled: A Brief Guide to the Standard Object Modeling Language (3rd ed.). Addison-Wesley Professional. In Chapter 9, "Use Case Diagrams," it is stated: "Actors are roles that people (or things) play when interacting with the system. ... Actors are outside the system" (p. 102).

3. Cockburn, A. (2001). Writing Effective Use Cases. Addison-Wesley Professional. In Chapter 2, "The Basic Use Case," Cockburn defines a primary actor as having a goal requiring the assistance of the system and notes that actors are "something outside the system" (p. 23).