Which of the following is an example of an external style variation?

An external style variation refers to distinct architectural features that can alter a building's exterior appearance or design. In this context, a dormer is a structural addition that protrudes from the slope of a roof, often to provide light and space to an upper level of a building. It is a design element that impacts the visual style and aesthetic of the structure, thereby classifying it as an external style variation.

On the other hand, a garage, bungalow, and foundation each pertain to different aspects of a property. A garage is primarily a functional space for vehicles, a bungalow is a type of housing style characterized by its single story and low profile rather than a variation on the exterior style, and a foundation is part of the building's structural base, not an external aesthetic feature. Thus, the dormer stands out as the best example of an external style variation, due to its impact on the overall appearance of the property.