Final answer:
Distinguishing between facts and opinions is the most important reading skill prerequisite to understanding an author's purpose.
Step-by-step explanation:
The most important prerequisite to understanding an author's purpose is distinguishing between facts and opinions. When reading, it is crucial to be able to identify information that is presented as factual, supported by evidence or research, versus information that is presented as someone's personal viewpoint or belief.
By distinguishing between facts and opinions, readers can better understand the author's intended message and purpose.