Choice "B" is correct. An employer/independent contractor relationship arises when an employer hires someone to do a job but does not have control over the manner in which the work is performed. In performing and audit, a CPA must have independence with regard to how the audit is performed. Thus, an employer/independent contractor relationship arises.
Choice "d" is incorrect. An employer has control over the manner in which an employee performs his work. An employer does not have control over the methods that a CPA uses to perform an audit. Thus, the CPA is an independent contractor rather than an employee.
Choice "c" is incorrect. Master/servant is older terminology for employer/employee. Thus, this choice is wrong for the same reason that choice b is wrong.
Choice "a" is incorrect. An employer is a principal; the employee is the agent. In an agency relatinship there must be both a principal and an agent. There cannot be a principal/principal relationship.