To achieve a speed of 4 m/s, the bottle must be dropped at m.

For an object dropped from rest, its speed after falling a height h satisfies v² = 2gh. Solving for height gives h = v²/(2g) = 4²/(2 × 9.8) ≈ 0.82 m. So the bottle must be dropped from 0.82 m, choice C.

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