Explanation:

- Option 1: from

- Not suitable here. "From" usually indicates the source or origin.

- Example: "Get the ink from the bottle."

- Option 2: in

- Commonly used to describe the medium or method used.

- Correct Answer: "Sign in ink" suggests using ink as the medium.

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- Option 3: with

- Generally used to indicate the instrument used.

- Example: "Sign with a pen" focuses on the tool, not the medium.

- Option 4: by

- Usually denotes the agent or means, often for actions.

- Example: "By doing so, you help," not fitting for ink usage.

Overall, option 2: in is the most appropriate for filling in the blank, effectively indicating the medium, "ink," in which the documents should be signed for preservation.