Which feature characterizes a flat head door?

A flat head door is specifically characterized by its design, which features a rectangular frame topped with a straight horizontal edge. This design is not only aesthetically straightforward but also practical, as it offers a clean, sleek look suitable for various architectural styles. The flat top allows for easy alignment with adjacent surfaces, making it a popular choice in both residential and commercial applications.

The other options refer to different door styles: a curved top is indicative of an arch-top design, while a round shape suggests a rounded or circular door, both of which differ significantly from the definition of a flat head door. The mention of multiple small panels points to a panelled door design, which also does not align with the characteristics of a flat head door. Thus, the clear identification of the flat head door by its straight horizontal top sets it apart from these other styles.