The main purpose of the Cisco SD-Access overlay design is to enable the virtualized fabric services that run over the physical underlay. Cisco describes the overlay as the logical network created on top of the routed underlay, using virtual networks to provide segmentation and policy separation. SD-Access uses LISP for endpoint-to-location mapping and VXLAN for data-plane encapsulation, allowing Layer 2 and Layer 3 services to be delivered across a stable Layer 3 transport. Option C is the best available answer because the overlay exists to integrate and carry SD-Access services such as virtual networks, endpoint mobility, macrosegmentation, and security group policy. Simplified troubleshooting and visibility are management and assurance outcomes, not the overlay's main technical purpose. High availability depends on the underlay topology, node redundancy, and control-plane design, but it is not the primary definition of overlay design.
Reference topics: SD-Access overlay, virtual networks, LISP, VXLAN, segmentation, fabric services.