Comprehensive and Detailed Explanation:
The correct answer is A - Disconnect / Refresh / Reconnect / Authenticate / Synchronize.
* Before refreshing a Salesforce sandbox that is connected to Marketing Cloud, you must first Disconnect the integration. Otherwise, the link between MC and Salesforce could break.
* Then, Refresh the Salesforce sandbox (refreshing rebuilds a copy of the production org).
* After the refresh, you Reconnect Marketing Cloud to the refreshed Salesforce sandbox.
* Authenticate again (OAuth) to ensure permissions and tokens are up-to-date.
* Finally, Synchronize objects and data.
# Why others are wrong:
* Other options either refresh before disconnecting or sync/authenticate in the wrong order, which could cause major connection errors.
Exact Extract from Salesforce Official Documentation:
Salesforce Help - Refresh Salesforce Sandbox with Marketing Cloud Connect:
"Disconnect Marketing Cloud from the Salesforce Sandbox before refreshing it. After refreshing, reconnect, authenticate, and synchronize your settings and data extensions." (Source: https://help.salesforce.com/s/articleView?id=sf.mc_co_sandbox_refresh.htm)
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Explanation
Before disconnectingthe sandbox instance and connecting the production instance in the Cloud Kicks business unit, NTO should remove any existing user mappings, attribute mappings, and tracking subscriptions that are associated with the sandbox instance. User mappings define which Marketing Cloud users can access which Sales Cloud users' data. Attribute mappings define which Sales Cloud fields are used as subscriber attributes in Marketing Cloud. Tracking subscriptions define which Sales Cloud objects receive tracking data fromMarketing Cloud sends. These settings should be removed before changing the CRM connection to avoid errors or conflicts. Additionally, NTO should update any query activities that reference the synchronized data extensions from the sandbox instance, as these will be different from the production instance synchronized data extensions. References:
https://help.salesforce.com/articleView?id=sf.mc_co_disconnect_a_salesforce_org_from_a_business_unit.htm&
https://help.salesforce.com/articleView?id=sf.mc_co_map_marketing_cloud_users_to_salesforce_users.htm&typ
https://help.salesforce.com/articleView?id=sf.mc_co_create_attribute_mappings_for_your_account.htm&type=5

Querying contacts from the Einstein Engagement Frequency data extension when creating the send can be used to limit the number of emails a subscriber receives to a maximum of one email every 14 days. The Einstein Engagement Frequency data extension contains information about how often each subscriber engages with emails and what is their optimal send frequency. By filtering the data extension based on the optimal send frequency, the send can exclude subscribers who have already received an email within the last 14 days.
References: https://help.salesforce.com/s/articleView?id=sf.
mc_as_data_view_einstein_engagement_frequency.htm&type=5