Final answer:
Avoiding smoking is the most effective intervention to slow down the progression of COPD.
Step-by-step explanation:
The intervention that has the greatest influence on slowing the progression of a COPD patient is avoiding smoking. Tobacco smoking is the major cause of COPD, and quitting smoking or not smoking at all is the single best way to prevent and slow down the worsening of COPD. This is because smoking leads to the breakdown of alveolar walls, reducing the surface area for gas exchange and causing chronic poor airflow.