Answer: The correct answer is :
A. natural rate of unemployment
Explanation: No real economy can have zero unemployment. For the proper and healthy functioning of a growing economy, some amount of unemployment is necessary. This unemployment is known as natural unemployment or natural rate of unemployment.
The natural rate of unemployment emerges from volitional economic forces. It is considered to be a combination of frictional and structural unemployment. Wherein, frictional means the kind of unemployment that occurs from the people who are in-between jobs or are looking for them, and structural unemployment means unemployment due to fundamental shifts in the structure of an industry or such sector which causes reorganization of people like technological change.
Such changes occur frequently in a growing economy and hence economists believe that this level of unemployment is natural and necessary for growth.