Which of the following best describes an actor in a use-case diagram?

In a use-case diagram, an actor is best described as a role that can either be a user or another system that interacts with the system being modeled. This concept is fundamental in understanding how different entities relate to and influence the functionality of the application. Actors represent external entities that engage with the system, initiating actions and receiving responses, which is crucial for identifying the system's requirements.

Actors can be human users (such as a customer or administrator) or systems that interact with the target system (such as an authentication service or a third-party API). This broad definition allows for a comprehensive understanding of the various interactions that can occur.

While other options may touch on aspects of use-case diagrams, they do not accurately encapsulate the concept of an actor. For instance, describing an actor as a type of software component that executes actions is too narrow and focuses solely on internal aspects rather than the external interaction role that an actor fulfills. Similarly, defining an actor as an interface does not capture the idea of an external role; an interface may be part of the system design but does not represent entities interacting with the system. Finally, stating that an actor is a method for validating functional requirements mischaracterizes the role; actors are part of the modeling process rather than a