Explanation:

- Section 35A of the Code of Civil Procedure (CPC) relates to imposing compensatory costs for false or vexatious claims or defenses in a legal proceeding.

- On Plaintiff: Costs can be imposed if the plaintiff has brought a false claim.

- On Defendant: Costs can apply if the defense made by the defendant is found false or frivolous.

- On Co-defendant: A co-defendant can also bear costs if they have been part of vexatious proceedings.

- All the above (Correct Option): Costs under Section 35A can be imposed on any party involved if they engage in false or vexatious litigation.

Answer: Option 4 - all the above.