Rapid elasticity is the characteristic of cloud computing that enables services to automatically scale resources up or down, depending on current demand. It ensures that the computing environment can adapt quickly to workload changes. The other options are attributes of cloud computing, but they do not directly refer to the ability to scale resources automatically to meet demand. High availability refers to systems that are continuously operational for a long duration, while shared resources refer to the pooling of resources to serve multiple consumers. Metered utilization pertains to the measurement of resource usage for billing or monitoring purposes.