Final answer:
Load testing is the process used to test an application's ability to handle high numbers of concurrent users, helping to identify performance limits and ensure reliability under peak loads.
Step-by-step explanation:
The process used to ensure that an application can handle very high numbers of concurrent users or sessions is D. Load testing. Load testing involves creating simulations to test how an application behaves under the stress of multiple users or processes at the same time. This method helps identify the application's performance limits, and determine whether it can sustain the peak user load without unacceptable delays or errors. Techniques like setting up virtual users and scenarios are used to mimic real-world usage patterns and stress the application to its limits.