Resource pooling serves as the biggest challenge to using DLP solutions to protect data at rest because data is spread across large systems, which are also shared by many different clients.
With the data always moving and being distributed, additional challenges for protection are created versus a physical and isolated storage system. Portability is the ability to easily move between different cloud providers, and interoperability is focused on the ability to reuse components or services. Reversibility pertains to the ability of a cloud customer to easily and completely remove their data and services from a cloud provider.

With the entire software platform being controlled by the cloud provider, the standardization of configurations and versioning is done automatically for the cloud customer. This alleviates the customer's need to track upgrades and releases for its own systems and development; instead, the onus is on the cloud provider.
Although licensing is the responsibility of the cloud customer within SaaS, it does not have an impact on compliance requirements. Within SaaS, development and maintenance of the system are solely the responsibility of the cloud provider.

Cloud services make exclusive use of virtualization, and systems change over time, including the addition, subtraction, and reimaging of virtual machines. It is extremely unlikely that the exact same virtual machines and images used in a previous audit would still be in use or even available for a later audit, making the tracking of changes over time extremely difficult, or even impossible. Elasticity refers to the ability to add and remove resources from a system or service to meet current demand, and although it plays a factor in making the tracking of virtual machines very difficult over time, it is not the best answer in this case.
Resource pooling pertains to a cloud environment sharing a large amount of resources between different customers and services. Portability refers to the ability to move systems or services easily between different cloud providers.

A reservation guarantees to a cloud customer that they will have access to a minimal level of resources to run their systems, which will help mitigate against DoS attacks or systems that consume high levels of resources. A limit refers to the enforcement of a maximum level of resources that can be consumed by or allocated to a cloud customer, service, or system. Both guarantee and assurance are terms that sound similar to reservation, but they are not correct choices.

Explanation
No matter what form they come in, "services" are obtained or purchased by a cloud customer from a cloud service provider. Services can come in many forms--virtual machines, network configurations, hosting setups, and software access, just to name a few. Hosting and servers--or, with a cloud, more appropriately virtual machines--are just two examples of "services" that a customer would purchase from a cloud provider.
"Customers" would never be a service that's purchased.