A union security agreement that requires an employee to join the union within a certain time after hire is called a:

The correct answer is Union shop. A union shop is a type of union security agreement where new employees are not required to be union members to be hired, but they must join the union within a specified period (e.g., 30, 60, or 90 days) after being hired as a condition of continued employment. This strengthens the union's membership and bargaining power.

• A Closed shop (A) requires employees to be union members *before* being hired, which is illegal in the United States under the Taft-Hartley Act.
• An Agency shop (C) requires non-union employees to pay union dues or a service fee to cover the costs of collective bargaining, but they are not required to actually join the union.
• An Open shop (D) does not require employees to join or pay dues to the union, even if a union is present in the workplace.