Migrating the Subscriber Key in Marketing Cloud to be the Service Cloud Contact or Person Account ID allows for a consistent identification of customers across both systems. This also enables data synchronization and cross-cloud features via Marketing Cloud Connect. Using Service Cloud as a central point to hold unique identifiers from all systems, including B2C Commerce CustomerNo and Customer ID, allows for a single source of truth for customer data and facilitates data integration and reporting across systems. Customer 360 Data Manager is not yet available for B2C Commerce, so option B is not possible. Sending the Marketing Cloud Subscriber Key to Service Cloud and B2C Commerce is not necessary if option A is implemented, so option D is not optimal. Reference:
https://help.salesforce.com/s/articleView?id=sf.mc_co_subscriber_key.htm&type=5
https://help.salesforce.com/s/articleView?id=sf.mc_co_data_integration_best_practices.htm&type=5
https://www.salesforce.com/in/blog/2020/04/what-is-salesforce-customer-360.html