What must be true for a Salesforce Opportunity to sync to Marketing Cloud Account Engagement?

 For a Salesforce Opportunity to sync to Marketing Cloud Account Engagement, the Opportunity must have a Contact Role that is syncing to a prospect in Marketing Cloud Account Engagement. A Contact Role is a way of associating a Contact with an Opportunity, and specifying their role and level of influence in the sales process. Marketing Cloud Account Engagement uses the Contact Role to link the Opportunity to the corresponding prospect, and track their engagement and influence on the Opportunity4. References: 4: Opportunity Contact Roles