If you delete a submission for the unlocked Mission C (as shown in the image below), it will work the same way as deleting a submission for a non-unlocked mission.
The participant will receive a notification that their submission has been deleted, the points will be removed, and Mission C will reappear in their “Remaining” Missions list in the app, allowing them to retry it.
Once they retry successfully, the points will be awarded again.
For instructions on how to delete a submission, you can refer to this article
What happens if I delete a submission from a Mission that unlocks further Missions?
For scenarios that occur when you delete a submission from a Mission that unlocks another Mission, please refer to this article.