77 of 100: Flip Efficiently!

A chef wants to flip all of the pancakes in a 5-pancake stack so that the darker side of each pancake is face-down. The chef uses a series of flips, where at each flip the chef inserts a spatula and simultaneously flips all the pancakes above it.

If the chef flips each stack in the most efficient way possible, which one of these three stacks of pancakes requires the most flips?

Note that a flip reverses the top-to-bottom ordering of the group of flipped pancakes, as well as the orientation of each pancake. The animation is just for illustration of the stack-flipping process; the stack shown isn't related to the answer choices.

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