Explanation:

- Option 1: Co-defendant

- A co-defendant is another defendant in the same case. They are usually involved directly and may have relief claimed against them.

- Option 2: Pro-forma defendant

- A pro-forma defendant is named in the suit but no relief is claimed against them. They are often included to comply with legal formalities.

- Correct Answer: Pro-forma defendant

- Option 3: Interveners

- Interveners are parties who voluntarily become involved in a lawsuit because they have an interest in the outcome. They are not original parties in the case.

- Option 4: None of the above

- This implies none of the other options are correct, which isn't applicable here.