When employee compensation legislation was introduced.

The question asks when employee compensation legislation was introduced.

• The concept of employee compensation legislation, particularly workers' compensation, began to emerge in the late 19th century. Germany introduced the first comprehensive workers' compensation law in 1884. Other countries, such as the United Kingdom, followed with similar legislation in the late 1800s. While these laws became more widespread and refined in the early 20th century, their initial introduction occurred in the 19th century.

The correct option is a.
$\boxed{\text{a. 19th century}}$