Step 1: Frame the question. The stem asks which intervention is most responsible for the falling morbidity of duodenal ulcer in recent years.
Step 2: Pick the dominant factor. Proton pump inhibitors transformed ulcer care by producing strong, durable acid suppression that heals ulcers quickly and prevents acid-driven complications. Alongside improved insight into perforated peptic ulcer, this drove down complication rates.
Step 3: Weigh the rest. H. pylori eradication mainly prevents recurrence and lifestyle change is a minor contributor for duodenal ulcer, so neither matches the emphasis of this recall answer as well as PPIs do.
\[\boxed{\text{Proton pump inhibitors}}\]