D. Volume-based: This model charges based on the amount of usage, such as the number of transactions, amount of data processed, or number of users accessing the application. Costs increase with higher usage and decrease with lower usage, making it the most likely model being described in the scenario.
Subscription: A subscription model charges a recurring fee, usually monthly or annually, for the use of the software. While this can sometimes be based on the level of service or number of users, it typically does not fluctuate directly with usage levels.