Dynamic branding in Experience Cloud relies on the Experience ID, sometimes carried as the expid parameter or equivalent placeholder in the URL, so Salesforce can render the right login experience for the selected brand. This is the scalable way to vary the look and feel without duplicating entire identity stacks. The community template choice can matter because not every template exposes branding capabilities the same way, but the central concept is still the Experience ID. External CMS tools are not required just to route the login experience by brand. From an architecture standpoint, this keeps the login framework centralized while letting the entry experience adapt to whichever brand, campaign, or sub-experience the user selected before authentication. This is why option D is the best answer in Salesforce terms.