**Core Concept:** Past Perfect tense usage **Analysis:** The Past Perfect tense (had + past participle) explicitly indicates an action completed before another past event. **Correct Answer:** B. The phrase 'completed before another past action' directly matches the definition of Past Perfect. **Error Analysis:** - A: Past Continuous describes ongoing actions - C: Present Perfect connects past to present - D: Simple Past indicates single completed actions **Common Mistake:** Confusing Past Perfect with Simple Past when sequencing events.