A dedicated Business Unit is the architectural answer whenever two brands must operate with separate identities inside a single Enterprise 2.0 account. Business Units partition Content Builder folders, Data Extensions, sending profiles, subscriber lists, and role permissions, so users working in one BU cannot see or accidentally reference assets belonging to another. This directly satisfies the requirement that the brands never mention each other, since content authors physically cannot pull assets across BU boundaries without explicit sharing. A Private Domain (B) only changes the sending domain used for links and tracking within the existing BU; it does nothing to separate content, styling, or subscriber data, and both brands would still share the same account-level assets. A second Sender Authentication Package (C) affects authentication (SPF/DKIM) and deliverability reputation for a domain, not content isolation or brand identity. An SSL Certificate (D) secures data in transit for landing pages and has no bearing on brand separation. Consultants should default to a new child Business Unit whenever contrasting audiences, governance, or branding requirements are described, reserving Private Domains and SAPs for deliverability and authentication conversations layered on top of that BU structure. Reference: Business Unit Hierarchy and Governance, Email Studio Setup.