Explanation:

- Section 91 of the Evidence Act deals with the admissibility of documents. It emphasizes the importance of written documents in providing evidence.

- Option 1: Transactions which under the law must be in writing

- This refers to any agreements or contracts that the law mandates should be documented in writing to be valid or enforceable.

- Option 2: Transactions which are reduced into writing voluntarily

- This involves instances where the parties choose to document their agreement in writing, even if the law doesn’t mandate it.

- Option 3: Both (a) & (b)

- This option correctly identifies that Section 91 applies to both legally required written documents and those created voluntarily.

- Option 4: Only (a) not (b)

- This suggests that Section 91 applies exclusively to mandatory writings, which is not entirely accurate.