Which of the following best describes a mansard roof?

A mansard roof is characterized by its unique design featuring two slopes on each side, with the lower slope being steeper than the upper slope. This design allows for the upper part of the roof to provide additional living space, often creating a usable attic area. The distinct two-pitch structure is what sets the mansard roof apart from other types of roofs.

When considering the other options, a roof with a single slope on each side describes a shed roof, which lacks the dual-pitch aspect of a mansard. A roof with triangular gables on each end refers to a gable roof, which does not have the complex slope structure of a mansard. Lastly, a flat roof without any slope would not be classified as a mansard roof at all, since a defining feature of the mansard is its pitched design. Thus, option B accurately reflects the essential characteristics of a mansard roof.