What are guilds?

organizations of slave owners

people who labor in the service sector

children of serfs who moved to the towns

organizations of artisans in the same trade

I believe the answer is: organizations of artisans in the same trade

Guilds were formed by a group of people who worked in the same line of business. The purpose is so they could minimalize competition, regulate price of their services, and work together to find larger projects that bring more profit for them.

A medieval association of craftsmen or merchants, often having considerable power. an association of people for mutual aid or the pursuit of a common goal.