Which of the following units is used to express resolution in imaging?

The correct choice represents how resolution in imaging is quantified. Resolution refers to the ability of an imaging system to distinguish between two closely spaced objects. "Line pairs per millimeter" is a direct measurement of this capability, determining how many pairs of lines can fit into a millimeter on an image. It is a widely accepted unit in fields such as radiography and other imaging disciplines, providing a clear standard for assessing image clarity and detail.

In contrast, while the line-spread function can describe how an imaging system responds to a point source of radiation, it does not provide a direct numerical measurement of resolution itself. The line-focus principle pertains to the geometrical concepts of x-ray tube design and does not express resolution directly. Thus, while the other options may relate to imaging concepts, they do not serve as definitions or units for expressing resolution like "line pairs per millimeter" does.