Login Discovery is the Salesforce feature built for identifier-first sign-in experiences that start with an email address or phone number and then route the user into the next authentication step. It is especially relevant for passwordless designs because the user can enter an email address or mobile number and then receive a verification code through the configured handler and verification service. That matches the stated requirement much more closely than social sign-on or a generic external IdP recommendation. The key platform point is that Login Discovery separates "who are you?" from "how do we authenticate you?" and allows Salesforce to branch appropriately. For citizen or customer portals, it is the standard entry point for phone-or-email-based identification before code verification. This is why option B is the best answer in Salesforce terms.