According to the Scrum Guide, two valid actions that the Developers can take when they realize that the workload may be greater than their capacity to complete the work are ensuring that the Scrum Team is aware, starting the Sprint, and monitoring progress, and removing or changing selected Product Backlog items. These actions are consistent with Scrum values and principles, such as transparency, adaptation, and collaboration.
The other options are not valid actions, as they are either wasteful (such as recruiting additional Developers or canceling the Sprint) or unsustainable (such as working overtime).