Which of the following describes a feature of cloud computing that offers flexibility in resource allocation?

Scalability is a fundamental feature of cloud computing that provides flexibility in resource allocation. It allows organizations to adjust their resources—such as computing power, storage, and network bandwidth—quickly and efficiently in response to varying demands. For example, during peak usage times, a company can increase its resources to accommodate higher traffic and then scale down during quieter periods, which ensures that they only use and pay for what they need. This dynamic adjustment contributes not only to operational efficiency but also to improved performance and user experience.

While the other options also represent important aspects of cloud computing, they do not specifically address resource allocation flexibility in the same way. Reliability focuses on the system's ability to operate consistently, security pertains to protecting data and resources from unauthorized access, and cost-efficiency relates to the budgeting and financial aspects of using cloud services. Scalability uniquely encapsulates the ability to grow and shrink resources as necessary, making it the most appropriate choice for describing flexibility in resource allocation.