Rapid elasticity is the characteristic of cloud services that allows systems to automatically scale resources up or down in response to fluctuating demand. This ensures that during peak times such as holiday sales, the e-commerce application can handle spikes in traffic without any manual intervention. High availability concerns the systems' ability to remain operational even during faults or maintenance, while metered utilization deals with tracking resource usage for billing. File synchronization is related to keeping files updated across systems but doesn't address scaling resources based on demand.