Final answer:
A personal trainer is not considered a doctor, unlike a physician, physical therapist, or lead medical researcher, who typically hold advanced degrees. Therefore, the correct option is c.
Step-by-step explanation:
With regard to academics, the person not considered a doctor is a personal trainer. A physician is a medical doctor, a physical therapist typically holds a master's degree or doctorate and is an expert in body mechanics, and a lead medical researcher often holds a PhD or similar degrees in their field of research. However, a personal trainer does not typically hold a doctoral degree in their field and therefore is not referred to as a 'doctor'.