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?
A composite key made from combining employee and department data
An additional primary key in the 'Employees' table for department data
A foreign key that references the 'Departments' primary key
A unique key in the 'Employees' table for each department