**Core Concept:** Subjunctive mood structure **Analysis:** The subjunctive uses base verb form after expressions of necessity. **Correct Answer:** C. 'Be informed' correctly follows 'crucial that'. **Error Analysis:** - A: Incorrect 'was' instead of 'were' - B: Missing past tense 'had' - D: Not a subjunctive context **Common Mistake:** Confusing subjunctive with conditional constructions.