Which of the following best describes a semicircular window?

A semicircular window is characterized by its shape, specifically having a semicircle at the top, which often sits above a rectangular or square section. This design allows for an arched appearance while maximizing the width at the base. The feature of the semicircle enables natural light to enter the space in a manner that is aesthetically pleasing and often complementing various architectural styles.

The other options describe different forms: a pyramid shape would not resemble any part of a semicircle, while a triangle on top defines a gable or peaked window. A circular window lacks the defining features of a semicircular design since it does not include the flat base that is typical in semicircular styles. Thus, the selection of the option that describes a window with a semicircle on top directly aligns with the fundamental characteristics of a semicircular window.