Had it not been for her quick thinking, the plan ____ into chaos.

"Had it not been for her quick thinking, the plan ____ into chaos."

The sentence begins with "Had it not been for...", which is an inverted (flipped) form of the third conditional — used for unreal past situations.

The non-inverted version: "If it had not been for her quick thinking, the plan would have descended into chaos."

The rule: "If + had (past perfect), would have + past participle."

So the answer is Option 2: "would have descended".

The sentence means her quick thinking saved the plan from falling apart. Since the chaos didn't actually happen, we use "would have descended" to describe the imaginary result.

A similar example: "Had it not been for the umbrella, we would have gotten wet" — we stayed dry because we had the umbrella!

Why not the others?

• Option 1: "would descend" → Used for unreal present/future situations, but "Had it not been" tells us this is about the past.
• Option 3: "had descended" → This is just past perfect. The result clause of a third conditional needs "would have," not "had."
• Option 4: "will have descended" → Future perfect — but the sentence is clearly about a past event.