Why did gender discrimination in the workplace become a major issue only relatively recently?

There were many reasons. For starters, women weren't allowed to work for a long time, up until the 20th century. When they did start working, gender discrimination was common and wasn't considered to be discrimination, it was the norm that came with the job. Only in recent times have people started understanding this and have started fighting gender discrimination in the workplace.

Ithink it is 4. making colonies. 

He issued the emancipation proclamation. it started changing slavery, and eventually lead to the freedom of slaves.