This answer fits because the requirement is about the specific platform mechanism named in the option, not a nearby workaround.
The Core Concept Explained: Experience Cloud design is driven by persona, authentication, scale, and sharing. The wrong template or license can create unnecessary cost, weak security, or excessive customization.
Step-by-Step Technical Analysis: Begin with the external persona, authentication requirement, and transaction volume. Select the Experience Cloud license or template that matches that access pattern, then configure sharing sets, external OWD, site visibility, and page access. Finish by testing the experience as anonymous, authenticated, and internal users. Set external organization-wide defaults to Private and configure Sharing Sets to grant access through the user's Account or Contact relationship.