What is the name of the triangular area below a roofline?

The correct choice refers to the gable, which is the triangular area formed at the end of a pitched roof, where two sloped sides meet. This architectural feature typically extends from the eaves to the peak of the roof, providing both structural support and aesthetic appeal. Gables can also serve to allow for ventilation in certain styles of roofing and help with the overall drainage of rainwater.

The foundation, on the other hand, refers to the lower portion of a structure that supports the building above, while the eave is the edge of the roof that overhangs the walls. The ridge is situated at the highest point of the roof where the two slopes meet. Each of these choices pertains to different aspects of a building's structure and design, but the gable is specifically identified as the triangular section below the roofline.