Answer:
A. trained doctors, physical therapists, and certified trainers
Explanation:
When you're listing three or more things, you should separate them with a comma. The last two words that make up the list should be connected by a conjunction, usually <em>and </em>or <em>or</em>. That conjunction should also be preceded by a comma. This type of comma is called an oxford comma, and its purpose is to make the sentence clear, unambiguous, and understandable.
Based on this information, we can conclude that the correct answer is option A.
