Circular mils can be expressed in which of the following measurements?

Circular mils is a unit of measurement used to describe the cross-sectional area of a wire, and it is specifically useful in electrical applications to ensure proper wire sizing for current carrying capabilities. This measurement is defined based on the diameter of the wire rather than the radius or square dimension.

When we refer to "circular mils," it is directly related to the concept of area, calculated by the formula ( \text{Circular Mils} = \left( \frac{D}{1} \right)^2 ), where ( D ) is the diameter of the wire in mils (one mil equals one-thousandth of an inch). Therefore, circular mils can also be expressed as an area unit in square mils, considering the fact that one circular mil corresponds to an area of one square mil.

Understanding that the circular mil is fundamentally a measurement of area, referring to it in the context of square mils accurately reflects its relationship to other area units. Thus, expressing circular mils in terms of square mils equivalent aligns with how this measurement operates within electrical engineering fields, making it the correct choice for this question.