If Salesforce is brokering identity from an enterprise SSO platform to downstream applications, then in its relationship with the enterprise SSO system Salesforce is acting as a service provider. It is consuming authentication from the enterprise identity source and then using that result to participate in further federation to third-party apps. That distinction matters because the same platform can be an SP upstream and an IdP downstream in the same overall architecture. Resource server and client-application labels do not describe the identity role Salesforce is playing in that specific trust relationship. The key point is directional trust:
Salesforce receives the authentication from the enterprise SSO provider, so it is the service provider in that connection. This is why option A is the best answer in Salesforce terms.