Which law established the principle of equal pay for equal work?

The correct choice is the Equal Pay Act of 1963, which specifically aimed to abolish wage disparity between men and women who perform equal work in the same establishment. This law establishes the principle that employees should receive equal pay for equal work, ensuring that gender does not influence salary levels for comparable roles. The act addresses situations where individuals undertake the same responsibilities with the same skills, effort, and working conditions.

The other options do not focus specifically on equal pay for equal work. The Family and Medical Leave Act protects employees' rights to take unpaid leave for family and medical reasons, but it does not address pay inequality. The Fair Labor Standards Act primarily regulates minimum wage and overtime pay, rather than specifically addressing issues of pay equality based on gender. The Americans with Disabilities Act focuses on prohibiting discrimination based on disability in various areas, including employment but does not specifically tackle wage issues.