What best describes the billing method for cloud services where clients only pay for what they use?

The billing method that allows clients to pay only for what they use is best described as pay-as-you-go. This approach is particularly advantageous for cloud services as it provides flexibility and cost-effectiveness, allowing clients to scale resources according to demand without incurring costs for unused capacity.

In pay-as-you-go scenarios, users are charged based on actual usage metrics, such as data storage or computing time, rather than fixed rates. This model is ideal for businesses with fluctuating workloads, as they can easily adjust their cloud consumption in response to changing needs while only being billed for those exact resources utilized during a given period.

On the other hand, the flat-rate billing method involves a fixed monthly or yearly charge that does not vary with usage, making it less suitable for variable consumption patterns. Annual subscriptions lock users into a set period, requiring a commitment upfront regardless of actual usage. Usage-based pricing might sound similar to pay-as-you-go, but it can imply a more complex pricing structure that incorporates various factors, while pay-as-you-go is typically straightforward and straightforward. Thus, pay-as-you-go is the best fit as it directly aligns with the concept of being charged solely for usage.