What legal concept that treated a collective group of people as a unit had evolved in Europe by the twelfth and thirteenth centuries?

The correct answer to the following question will be "The corporation".

Explanation:

The term corporation originated from either the word corpus (Latin word) for the entity which represents a group of persons permitted to function as just an individual. It's a separate legal entity and is different from its members. By the twelfth and thirteenth centuries, this idea viewed a single group of individuals as a body that had emerged in Europe.

Corporations retain the bulk of corporations rights and privileges:

They may enter into contracts. Lend and take out loans. Prosecute and sue. Pay taxes. Hire new employees and staff.

Therefore, it's the right answer.