Explanation:

- Section 91 of the Indian Evidence Act: Focuses on the proof of the contents of documents. It stipulates that when a document is required by law to be in writing, the document itself must be produced for the inspection of the court.

- Unilateral Documents: These are documents that involve obligations or terms affecting only one party, like a will or a promissory note.

- Bilateral Documents: Involve two parties and are often contractual, like sales agreements or leases.

- Tripartite Documents: Involve three parties, often seen in more complex agreements like some financial transactions or business mergers.

- Correct Answer: Option 4 - All the above. Section 91 applies to all types of documents, whether unilateral, bilateral, or tripartite.