Why were guilds developed?

a. to encourage trade between regions
b. to establish a method for teaching skills
c. to protect the interests of the craftspersons
d. to fine those who typically produced poor work

Guilds were developed C. to protect the interests of the craftpersons.
Since they didn't have many opportunities at the time, they had to create some kind of a union where they could be given many chances and where their interests would be represented. Thus, guilds were founded.