Answer:
Extensive black migration to the North
Explanation:
From 1916-1970, 6 million African Americans migrated from the Southern states to the North, mostly for social reasons (escape from racial hatred, evade Jim Crow laws, meet up with black communities in the North) and economic opportunities (more factory jobs). This is known as The Great Migration.
The other answers can be eliminated since (and I'm not 100% sure about this one, but assume that) the huge increase in factory workers for the war effort in World War 1 meant that trade unions became more appealing to workers so they could negotiate wages and benefits with industry managers. After all, the purpose of a trade union is to protect the interests and rights of the workers, so trade unions likely did not decline.
"Loosing of controls on freedom of speech" is incorrect because of the Espionage Act of 1917, where the government could imprison you for disagreeing with them, like when Socialist Party leader Eugene Debs was sentenced to 10 years for saying the wrong things.
And for the last answer: the number of women in the workplace EXPLODED after the U.S. entrance into WWI; since so many men were sent to Europe, someone had to take their place at work, especially in factories that contributed ammo and other supplies for the war effort.
