Socialism was a reaction against what?

The replacement of aquaculture by industry?

The inequality between owners and workers?

The growing importance of international trade?

The establishment of republics with constitutions?

Socialism was a reaction against the inequality between owners and workers.

The answer is surely right. The reason why it's the correct answer is because the increasing divide between owners and workers started increasing more and more resentment towards the owners and therefore caused an increasing desire for a more egalitarian society.