What is concept of agency? Which of the following is correct?

An agent is any person who calls themselves an agent, whether or not they work for themselves or as an employeeA person can only be an agent if they register as an agent, e.g. a real estate agentAgents are anyone who are employed to carry out work for anotherAn agent is a person who has been authorised to act on behalf of another (the principal) whereby that agent creates a legal relationship with a third party which is binding on the principal