Explanation:

- Option 1: Retrospective in operation

A mandatory injunction typically requires a party to undo something already done, which implies a retrospective effect, at times.

- Option 2: Prospective in operation

Mandatory injunctions often command actions to be taken in the future, aligning with prospective operation.

- Option 3: Concurrent in operation

Injunctions can coincide with existing conditions that need correction, giving them a concurrent aspect.

- Option 4: All of the above

Correct Answer: A mandatory injunction can be retrospective, prospective, or concurrent, depending on the case's specifics.