Choice "C" is correct. Structural unemployment occurs when the jobs available do not match the skills of the unemployed individuals or when the individuals do not live where jobs are available with their skills.Choice "b" is incorrect. Frictional unemployment exists when workers are in the process of changing jobs or are temporarily laid off from their jobs.Choice "d" is incorrect. The natural unemployment rate is the sum of frictional, structural, and seasonal unemployment or the unemployment rate that exists when the economy reaches its potential output level.Choice "a" is incorrect. Cyclical unemployment is due to a downturn (recession) in the economy which leads to a decline in real GDP and higher unemployment.