Which of the following indicates slowing down when backing?

Arms out to the side is a commonly understood signal that indicates a need to slow down when backing up. This gesture creates a visual representation of caution and awareness, making it clear to others that the person operating the vehicle should reduce their speed or proceed with care. In many contexts, this signaling helps maintain safety by ensuring that those around the vehicle understand the operator's intentions.

Other signals such as crossing arms in an X or raising arms up and down might convey different meanings that are not specifically about slowing down. Gabbling both hands together doesn't convey a clear signal in a backing context, making it less effective than extending arms out to the side. Overall, the gesture of arms extended sideways is widely recognized and serves as a clear communication tool in the context of backing up safely.