#Comprehensive and Detailed Explanation:
Option B uses a Salesforce CRM Date-Based Entry Source in Journey Builder, which is designed exactly for these kinds of time-based communications triggered off of Salesforce object records (such as Appointment__c). This entry source allows Marketing Cloud to initiate a Journey X days before a date field, and execute native Salesforce activities (such as updating the record when the message is sent).
You can enter contacts into a journey before or after a specific date in a CRM object.
You can configure the journey to start at a specific time of day, such as 10:00 AM.
You can use the Salesforce Update Contact Activity within the Journey to update fields on the original record.
Exact Extract from Salesforce Documentation:
#Source: Salesforce Help - Use a Salesforce Data Entry Source in Journey Builder
"Date-Based Entry: Add contacts to a journey based on a date attribute in the Salesforce object. You can configure contacts to enter the journey before, after, or on the specified date and time."
"The Salesforce Update Contact Activity updates a record in your connected Salesforce account. For example, use the activity to change the status of a lead or add a note to a contact after the contact receives an email."
#Source: Marketing Cloud Connect Implementation Guide
"With Marketing Cloud Connect, you can use Salesforce Data (such as Contacts or Custom Objects) as the entry source in Journey Builder. This allows for triggering journeys directly from CRM records with date- based logic."
#Why Other Options Are Incorrect - Backed by Official Extracts:
A - Invalid due to write-back to CRM limitations
"Synchronized Data Extensions are read-only copies of Salesforce CRM data. They can be used to segment and send, but not updated directly from Marketing Cloud."
- Source: Salesforce Help - Synchronized Data Extensions
C - Not ideal due to manual AMPscript update workaround
"AMPscript can interact with data extensions, but using it to write back to Salesforce CRM requires custom API integration and is not recommended when using Marketing Cloud Connect." D - Data Extension entry source doesn't natively update CRM records
"Journeys triggered from a data extension cannot natively update Salesforce CRM unless custom API calls are made through custom activities or server-side scripts."