High availability is the correct answer as it ensures that computing resources are available and operational to the client without noticeable downtime. By prioritizing high availability, the company will choose a cloud service provider that has redundancy and failover mechanisms built-in to serve the web application even during failures or maintenance periods. Other elements like shared resources, metered utilization, and file synchronization are beneficial but do not specifically address the need for constant application uptime, making them incorrect in this context.