How Are "Best Practices" Produced and Identified

How are "best practices" produced and identified?

A) Outcomes studies are conducted to verify clinical observations and their findings systematically distilled into guidelines to inform practice.
B) Experts in the field document their clinical observations, which are published and used to create guidelines to inform practice.
C) Outcomes studies are systematically ranked by the experts in the field, based what they have clinically observed as significant.
D) Experts in the field are surveyed to find out what practices they are using.