Rapid elasticity is the correct answer because it specifically refers to the ability of cloud services to automatically scale resource capacity up or down as needed, in a timely manner. This ensures that the online retail company can accommodate the increased workload during their sale without manual adjustment of resources. High availability is more about ensuring the continuous operation of services rather than automatically scaling resources. On-demand self-service allows users to provision resources as needed, but does not imply the automatic scaling characteristic of rapid elasticity.