In a labor union, laborers
must work day shifts
must be under eighteen years of age
must work in cotton mills
work together in negotiating for better wages and conditions

The correct answer is - work together in negotiation for better wages and conditions.

The laborers in a labor union are working together and support each other for the interest of all of them sot hat they can get better wages and better working conditions. This is also important because they work as a unit, so the owners do not have the upperhand and have to take in consideration their requirements. The labor unions are very important for the protecting the rights of the workers as well, so that they are not under threats, or forced to do anything they are not supposed to.