Universal Containers (UC) has a private sharing model for Opportunities and uses Opportunity teams.

In Salesforce, there are different types of sharing: Managed Sharing (Role Hierarchy, Sharing Rules) and Manual Sharing. A critical behavior of the platform is that when a user is deactivated, all their Manual Shares (records shared with them by other users using the "Share" button) are automatically and permanently deleted from the system. Even if the user record is reactivated later, those manual shares do not return. Because the organization uses a "Private" model and does not use criteria-based sharing rules, the user's previous access likely relied on manual sharing or their previous position in the hierarchy. While activating the record and adding them back to teams provides new access, the historical "one-off" shares are gone. Options A and B are unlikely to be the cause of a total loss of visibility. Option D is incorrect because permission sets control what a user can do, not which specific records they can see in a private sharing model.