Which of the following is not a covered entity under HIPAA?

Under HIPAA, covered entities are defined as health plans, healthcare providers who conduct certain transactions electronically, and healthcare clearinghouses that process health information. Fitness studios typically do not engage in activities that involve the electronic transmission of health information for the purposes specified by HIPAA. Therefore, they are not considered covered entities, as their primary functions do not revolve around providing healthcare services or managing health information in a way that would invoke HIPAA regulations.

In contrast, health insurance companies, healthcare providers, and healthcare clearinghouses all handle or transmit protected health information, making them subject to HIPAA compliance. This is why fitness studios stand out as the correct answer in this context.