When partner onboarding must prevent duplicate accounts across Salesforce and an external identity store, the safest design is to control registration in one orchestrated flow rather than allowing each system to create identities independently. A custom self-registration experience can validate whether the partner already exists, create the Salesforce partner records in the right account structure, and coordinate identity creation only once. This avoids the common duplication problem where the identity provider and Salesforce both accept separate registrations for the same business user. The architecture goal is not simply sign-in; it is controlled provisioning with deduplication. For that reason, a custom registration page or process that checks existing partners and then creates records in the right order is the strongest fit. This is why option A is the best answer in Salesforce terms.