Exam Sharing-and-Visibility-Architect Topic 1 Question 226 Discussion

Explanation
The most likely cause of this problem is that the sales rep was manually removed from the Opportunity team.
This would revoke the access to the opportunity record that the sales rep was helping with. The other options are not valid causes, because:
The Account team does not affect the Opportunity team members, unless the account owner changes and the opportunity owner is set to match2.
The Opportunity team is specific to each opportunity record, so removing the sales rep from one opportunity team does not affect the access to another opportunity record of the same account3.
The opportunity owner cannot enable/disable if the "Default Opportunity team" is able to access the record.
The default opportunity team is a template that can be applied when creating or editing an opportunity, but it does not override the existing opportunity team members.