Answer: d. none of the choices.
Explanation:
Joining a union is a voluntary act which means that if a person decides not to join one, not much can be done about it by the Union.
The Union cannot compel the company to discharge or fire the employee, neither can they encourage them to deny the employee a promotion or discriminate against the employee in their employment of the employee.
The main disadvantage of not being in a trade union is that they will not represent you in disputes against the company.