Choice "C" is correct. When the economy is operating at full employment, there is no cyclical unemployment. When the economy is operating at full employment, there is still some unemployment known as the natural rate of unemployment, which does not include cyclical unemployment.
Choice "d" is incorrect. There is still some unemployment (frictional, structural, and seasonal) when the economy is operating at full employment.
Choice "a" is incorrect, per the above explanation.
Choice "b" is incorrect, per the above explanation.
