Which type of column is characterized by a flat rectangle on top, resembling a door?

The correct answer is Doric. Doric columns are known for their simplistic and sturdy design, often used in ancient Greek architecture. The top of a Doric column, known as the echinus, is often flat and lacks elaborate decoration, giving it a more austere appearance compared to other column styles. The flat rectangular shape can indeed resemble a door, which aligns with the description provided in the question.

In contrast, Ionic and Corinthian columns are characterized by more ornate capitals. Ionic columns feature volutes, which are scroll-like elements on their capitals, while Corinthian columns have an elaborate leafy design. Composite columns combine elements of both the Ionic and Corinthian styles but do not have the flat rectangle characteristic of Doric columns. Thus, the simplicity and structure of the Doric column make it the fitting answer in this context.