Universal Container's sales reps can modify fields on an opportunity until it is closed.

To manage field editability based on the opportunity status:

C. Use field-level security on page layouts with record types to restrict editing fields. This combination allows for different layouts and editable fields based on the status of the record (e.g., closed or open).

Steps to implement:

Create or adjust record types for open and closed opportunities.

For each record type, create a specific page layout.

On the page layout for closed opportunities, set the majority of fields to read-only using field-level security, except for the 'Post-Close Follow-up Date' and 'Post-Close Follow-up Comments' fields.

Assign the appropriate page layouts to the respective record types.

Update profiles or permission sets to use these record types and page layouts accordingly.

This setup ensures that sales reps can modify fields only when the opportunity is open, and the sales operations team can edit specific fields after closure.

For more information on using record types and page layouts, check Salesforce's documentation on Record Types.