A company has two tables in their database: 'Employees' and 'Departments'. Each employee is assigned to one department. To ensure that the 'Employees' table references the correct department data, what kind of special field should be included in the 'Employees' table?
An additional primary key in the 'Employees' table for department data
A composite key made from combining employee and department data
A unique key in the 'Employees' table for each department
A foreign key that references the 'Departments' primary key