Hi there!
The answer is A) A lack of labor mobility as people choose to remain in their hometown.
Unemployment is a complex issue, here are the reasons why the other options increase unemployment:
-Reduction in Union bargaining activities would increase unemployment because employers wouldn't have as many restrictions to reduce their workforce.
-Reduction in economic activity due to a recession would increase unemployment as the demand for products, and for workers too, will decrease.
-The existence of efficiency wages causes unemployment because the demand for labor will decrease, as employees get more and more efficient.
A lack of labor mobility as people choose to remain in their hometown will not contribute to a higher level of unemployment. Instead, labor mobility can increase unemployment because there would be an oversupply of labor in a specific region.