What kind of role can an actor take that features them in a very singular role with a specific purpose, usually only for a limited number of scenes?

The correct choice is "Guest star." In the context of film and television, a guest star is defined as an actor who plays a significant yet limited role within a particular episode or a specific segment of a project. This role often has a clear purpose, contributing to the storyline or character development, but is generally not as extensive as that of a regular cast member.

In contrast, supporting roles typically refer to actors who appear throughout the main narrative and provide essential character interactions but are not the primary focus of the story. Lead roles are those where the actor is central to the plot, carrying much of the narrative weight across the film or series. Background roles, often referred to as extras, involve actors who provide atmospheric context without specific dialogue or substantial character development. Each of these roles plays a vital part in storytelling, but the defining aspect of a guest star is the impactful yet temporary nature of their involvement in a production.