Natural rate of unemployment - Wikipedia, the free encyclopedia
Measuring the rate operationally (if that can even be done) will presumably depend on a technical definition, and there may be more than one in the literature.
I am guessing, however, that the assumption behind this question is that unemployment is at its "natural rate" when the economy is producing at its average long-term potential. Whether that assumption has any empirical validity is a different question. See if there is anything in the text that seems consistent with that assumption.