Explanation
Any application can be a possible target of denial-of-service (DoS) attacks. From the application side, the developers should minimize how many operations are performed for non-authenticated users. This will keep the application running as quickly as possible and using the least amount of system resources to help minimize the impact of any such attacks.

The infrastructure service capability gives the cloud customer substantial control in provisioning and configuring resources, including processing, storage, and network resources.

Homomorphic encryption hopes to achieve that goal; the other options are terms that have almost nothing to do with encryption.

Explanation
KPIs fall under the "business" aspect of QoS, along with monitoring and measuring of events and business processes. Services, security, and applications are all core components and concepts of the "IT" aspect of QoS.