The choices can be found elsewhere and as follows:
A) a greater percentage of work force is unionized.
B) The minimum wage is higher than the market wage.
C) unemployment insurance benefits are paid for fewer weeks.
D) Firms pay efficiency wages.
I believe the correct answer is option B. The natural rate of unemployment tends to be lower when the the minimum wage is higher than the market wage. Hope this answers the question. Have a nice day.