Final answer:
The event that most directly contributes to the theme that the past cannot be revisited is the protagonist's realization that they can never return to their cherished memories, symbolized by 'the closing of the door'.
Step-by-step explanation:
The event in the poem that most directly contributes to the theme that the past cannot be revisited is the event described where the protagonist experiences a profound sense of loss and recognizes that they can never go back to the past. This realization is encapsulated in the passage where the character reflects on the irrevocable changes in their life, acknowledging that the dream was gone, and they were unable to revive the cherished memories of the past. The illustration of the closed gates and the metaphor of the gray beauty of steel exemplify the permanence of that loss and the impossibility of returning to the bygone days of 'winter dreams'. Out of the four options provided, the closing of the door best represents this notion, as it symbolizes the finality of the past being out of reach.