Which would provide the board with the highest level of assurance regarding whether an internal...

The IIA requires internal audit functions to have an external quality assessment at least once every five years. External quality assurance feedback provides the most independent, objective, and reliable assurance to the board regarding whether the internal audit function conforms with standards and is positioned to achieve its objectives.

Option A measures productivity but not quality. Option B provides useful insights but is subjective. Option D indicates staffing health but does not directly assess ability to meet objectives.