How should the diameter of a circle be represented in dimensioning?

In dimensioning a circle, the most common and accepted practice is to represent the diameter alone. This is because the diameter provides an immediate understanding of the circle's size, which is essential for manufacturing and construction purposes. When indicating the diameter, it is typically marked with a diameter symbol (a "D" or a slash through the dimension line) to clarify what measurement is being conveyed.

While other methods, such as showing both diameter and radius or using radius alone, may give additional information, they are not standard practice for clear dimensional communication in technical drawings. Radius measurements can lead to ambiguity when the intent is to convey the size of a circle directly. Similarly, dimensioning by circumference is rarely used because it does not provide an intuitive sense of the circle's dimensions compared to the straightforward diameter. Therefore, option A effectively conveys the necessary and clear information about the size of the circle.