B) Service Cloud should be implemented before Marketing Cloud to ensure that the Salesforce Contact IDs can be used in Marketing Cloud as unique identifiers. This allows for a consistent and unified view of the customer across the Salesforce products and avoids duplicate contacts in Marketing Cloud. C. Customer data from B2C Commerce and the legacy CRM should be deduplicated before being imported into Service Cloud. This ensures that there are no duplicate contacts in Service Cloud that could cause data quality issues or conflicts when syncing with Marketing Cloud. D. Custom development work is required to connect B2C Commerce to Marketing Cloud and use the Salesforce Contact ID. This involves creating a service definition in B2C Commerce that points to Marketing Cloud's REST API and passing the Salesforce Contact ID as the contact key when triggering journeys or updating contact data. Reference: https://help.salesforce.com/s/articleView?id=sf.mc_co_subscriber_key_migration.htm&type=5 https://developer.salesforce.com/docs/commerce/sfra/marketing-cloud/marketing-cloud-integration