At any given instant, a spinning probe eddy current instrument should be inspecting:

The correct choice is significant because a spinning probe eddy current instrument inspects within a defined area that corresponds to the size of the probe coil. This area represents the specific region of the material where the eddy currents are generated and are indicative of the material's properties or any defects present.

When the probe spins, it effectively scans the surface and collects data over this defined area, allowing for accurate evaluation of the material's integrity. The size of the probe coil directly influences the resolution and the sensitivity of the inspection; thus, the instrument is designed to utilize this specific area for effective measurement and analysis.

Understanding this aspect is crucial, as it highlights how the geometry and design of the eddy current probe impact inspection capabilities, focusing on localized areas rather than broader or more abstract references such as "one complete circumference" or "one longitudinal line."