At the last minute, Amidu … of the plan.

Analyze the question: The sentence says that "at the last minute" Amidu did something regarding "the plan." We need the phrasal verb that fits with "of the plan."

Explain the concept: "Back out of" is a phrasal verb meaning to withdraw from an agreement, plan, or commitment you had previously agreed to. It is always followed by "of" when an object is stated.

Check each option:

• A. backed up: means to support or to reverse/make a copy. "Backed up of the plan" is ungrammatical because "up" does not take "of."
• B. backed for: not a standard phrasal verb in this sense. "Backed for" does not work with "of the plan."
• C. backed out: "Amidu backed out of the plan" means he withdrew from it. It is grammatical, and the meaning fits "at the last minute."
• D. backed in: not a standard idiom, and it doesn't take "of."

Answer: The sentence becomes: *At the last minute, Amidu backed out of the plan.

Tip: Whenever you see a phrasal verb followed by "of," check for a pair like back out of, run out of, get out of, or get rid of. The preposition after the blank often tells you which particle is needed. Also remember: back out of = withdraw from.