Explanation:

- Interrogatories in legal procedures refer to a set of formal questions posed by one party to another to gather information essential for the case.

- Under Order XI, Rule 8 of the Civil Procedure Code (CPC), responses to interrogatories must be given.

- On an affidavit: This is the formal process where answers are given under oath in writing and are signed and sworn.

- On a simple application: This isn't correct for responses; it's more about requesting something from the court.

- By filing the documents: This involves submitting relevant documents but isn't how interrogatory answers are given.

- All the above: This is incorrect as applying, documenting, and affidavit are not simultaneously used for interrogatories.

The correct answer is:

- Option 2: On an affidavit