The most significant real economic cost of high unemployment is: the potential goods and services that might have been produced but weren't. the money cost of retraining persons to obtain new jobs. the lost tax revenue that might have been paid by persons if they had worked. the money cost of unemployment insurance payments to the unemployed.

the potential goods and services that might have been produced but weren't.

Explanation:

Unemployment occurs when those who are willing and able to work do not have jobs

Types of unemployment

structural unemployment is an unemployment that occurs as a result of changes in the economy. These changes can be as a result of changes in technology, polices or competition. Structural unemployment tends to be permanent.  

Frictional unemployment:  the period of time a person is unemployed from the period he leaves his current job and the time he gets another job. Eg. when a real estate agent who leaves a job in Texas and searches for a similar, higher-paying job in California.

Voluntary unemployment: e.g. worker at a fast-food restaurant who quits work and attends college.

Cyclical unemployment: it occurs as a result of fluctuations in the economy. Unemployment would be high in a downturn and low in a boom  

Economic cost or implicit cost or opportunity cost is cost is the cost of the next best option forgone when one alternative is chosen over other alternatives.

One of the most significant economic cost of unemployment is the potential output lost as a result of unemployment

definitely a