Which of the following is a MUST for creating a new custom-built, cloud-native application designed to be horizontally scalable?

NaaS delivers virtualized network services (bandwidth, connectivity) on demand but does not provide an application development platform, runtime, or scaling framework needed to build and run custom cloud-native applications.

PaaS provides developers with a managed platform including runtime environments, auto-scaling capabilities, load balancing, and orchestration services (e.g., Azure App Service, Google App Engine, AWS Elastic Beanstalk) that are specifically designed to support cloud-native application development and horizontal scalability. It abstracts infrastructure management so developers focus on application code, and built-in scaling mechanisms allow the platform to automatically add instances in response to demand. This makes PaaS the essential foundation for custom-built, horizontally scalable cloud-native apps.

IaaS provides raw compute, storage, and networking resources (e.g., VMs), but requires the developer to manually configure OS, middleware, scaling policies, and orchestration, making it insufficient on its own as a 'must' for cloud-native horizontal scalability.

SaaS delivers fully managed, ready-to-use software applications to end users and does not provide a development or deployment platform for creating custom-built applications.