Business continuity and disaster recovery (BCDR) solutions largely sit idle until they are actually needed.
This traditionally has led to increased costs for an organization because physical hardware must be purchased and operational but is not used. By using a cloud system, an organization will only pay for systems when they are being used and only for the duration of use, thus eliminating the need for extra hardware and costs. Portability is the ability to easily move services among different cloud providers.
Broad network access allows access to users and staff from anywhere and from different clients, and although this would be important for a BCDR situation, it is not the best answer in this case.
On-demand self-service allows users to provision services automatically and when needed, and although this too would be important for BCDR situations, it is not the best answer because it does not address costs or the biggest benefits to an organization.

Explanation
Because the public cloud model is available to everyone, in most instances all a customer will need to do to gain access is set up an account and provide a credit card number through the service's web portal. No additional contract negotiations, agreements, or specific group memberships are typically needed to get started.

With SDN, the filtering and forwarding capabilities and administration are separated. This allows the cloud provider to build interfaces and management tools for administrative delegation of filtering configuration, without having to allow direct access to underlying network equipment.
Firewalling and protocols are both terms related to networks, but they are not components SDN is concerned with.