Section 91 of Evidence Act applies to documents which are

Section 91 of the Indian Evidence Act applies to bilateral documents. In this context, bilateral documents refer to documents that involve two or more parties, typically in the form of an agreement, contract, or communication between two parties. Section 91 of the Indian Evidence Act allows for the production of such documents in court as evidence when their contents are required to be proved.

The other options do not accurately describe the scope of Section 91:
Option A: Unilateral documents involve only one party and are not typically covered by Section 91.
Option C: Tripartite documents involve three parties, but the section is concerned with bilateral documents.
Option D: All the above is not correct as Section 91 specifically applies to bilateral documents.

So, the correct answer is Option B: Bilateral, which accurately reflects the scope of Section 91 of the Indian Evidence Act.