Which of the following should all rescuers be tied into during a litter rigging operation?

In a litter rigging operation, all rescuers should be tied into mainline ropes. This is essential for ensuring that they are securely anchored and able to manage the load effectively while performing the rescue. Mainlines provide the primary means of support and stability, allowing for safe maneuvering of the litter and securing the rescuers during the operation.

Proper attachment to the mainline helps prevent falls and enhances the overall safety of both the rescuers and the victim being lifted or lowered. It's integral in situations where there is a risk of movement or instability, as it maintains a direct connection to the belay system or other safety measures in place.

Secondary harnesses, rescue bags, and safety helmets serve important functions in a rescue scenario but do not provide the same level of anchorage and security that mainline ropes do. The secondary harness is often a backup, the rescue bag might contain tools and equipment, and helmets are primarily for head protection, but none of these contribute to the core safety measure of being secured to the mainline during this critical rigging operation.