Amadi's plan … at the last moment.

Analyze the question: The sentence is "Amadi's plan _ at the last moment." We need a phrasal verb that means the plan failed.

Explain the concept: This is a phrasal verb question. Adding a different preposition or adverb to "fell" changes the meaning completely.

• fell down: to drop to the ground, or to collapse physically (e.g., "The old building fell down").
• fell on: to land on something, or to attack (e.g., "The soldiers fell on the enemy").
• fell through: to fail or come to nothing, usually of plans, arrangements or deals.
• fell off: to drop from something, or to decline in amount (e.g., "Sales fell off").

Solution:

1. "Plan" is an abstract noun, so a literal physical fall (fell down, fell on) doesn't fit.
2. "At the last moment" suggests that something that was arranged did not happen.
3. Fell off usually refers to a decrease or to dropping from something, which does not suit a plan.
4. Fell through means "failed to happen," which fits perfectly.

Completed sentence: "Amadi's plan fell through at the last moment."

Tip: Think of a plan as a bridge or a floor. If it falls through, it collapses and you never get to the other side. Whenever you see plan, deal, arrangement or negotiation with a failure meaning, choose fall through.