Which federal employment law makes it illegal for an employer to discriminate against an employee based on her or his union affiliation or activities?

The National Labor Relations Act is the federal employment law that specifically protects employees' rights to engage in union activities without facing discrimination from their employer. This law was enacted to encourage collective bargaining and protect the rights of employees to organize and join unions. It makes it unlawful for employers to interfere with, restrain, or coerce employees in the exercise of their rights to self-organization, union involvement, and collective bargaining. This includes protection against discrimination based on an employee's union affiliation or their participation in union activities, thereby ensuring that employees can freely advocate for their interests and work together without fear of retaliation.

The other laws mentioned do not focus on union-related protections. The Fair Labor Standards Act primarily addresses wage and hour issues; the Family and Medical Leave Act provides job-protected leave for qualifying family and medical reasons; and the Worker Adjustment and Retraining Notification Act deals with specific requirements around notifying workers about plant closings and mass layoffs. Thus, they do not provide the same level of protections concerning union activities as the National Labor Relations Act does.