The Opportunity Team feature is the most effective way to collaborate on an opportunity in a private sharing model. By adding the product specialist and solution engineer to the Opportunity Team, the junior account manager can share specific permissions for the opportunity without altering ownership. Additionally, Opportunity Team settings remain intact even if the account ownership changes to a senior manager.

Option A: Manual sharing is less flexible and requires re-sharing if the account is reassigned.

Option B: Sharing the account doesn’t ensure access to opportunities since sharing is record-specific in a private model.

Option C (Correct): Opportunity Teams allow role-based collaboration and persistent sharing, ensuring access is unaffected by ownership changes.