A new Marketing Cloud (MC) customer wants to now implement a Sales Cloud instance to...

Introducing Sales Cloud alongside an existing Marketing Cloud instance that has been keyed on Email Address for a year creates a structural identity mismatch, because Sales Cloud-driven integrations (Marketing Cloud Connect, Synchronized Data Extensions) key subscriber/contact records on the Salesforce record ' s own unique identifier, not raw email address — so without remediation, the same real-world person can end up represented as two disconnected records once the Salesforce integration goes live. Uploading a CSV that maps and updates existing Marketing Cloud records to use properly migrated Subscriber Keys aligned to their future Salesforce identifiers (B) directly resolves this by re-keying the existing base before Salesforce data starts flowing in, preventing a second, duplicate record from being created for the same person. Alternatively, engaging a dedicated services/migration effort to have existing records comprehensively updated with new keys sourced from the Sales Cloud instance (D) accomplishes the same de-duplication goal through a structured, professionally-managed remediation project. Option A is incorrect — Marketing Cloud does not natively ' re-key ' or deduplicate existing Email-Address-keyed records simply because Sales Cloud is later introduced; without remediation, duplication risk remains. Option C, purging historical records, is a destructive approach that loses valuable existing subscriber history rather than preserving and correctly re-keying it. Reference: Subscriber Key Migration, Sales Cloud Integration De-Duplication.