Answer:
b. firm that requires union membership by its workers as a condition of employment
Explanation:
A closed shop also called a pre-entry closed shop refers to an agreement whereby the employer only hires an employee if he has union membership and must remain a member of that union to continue in employment. It is a form of security agreement to ensure genuiness of employee. Closed ship is opposed to post-entry closed shop or union shop which requires the employee to become a member of a union if they are not already members, and must be accepted by the union.