Which best explains how unions help workers get better wages?

B

Explanation:

The union is founded and developed by a group of people with the aim to protect the benefit of workers. The reason why union can help labour raise their wage is that the appearance of union helps foster the inequality reduction in the working environment between low and middle wage workers and high wage workers.

So that workers join unions because the union is formed to bargain for better working conditions and higher wages.

the answer is d.index