The answer is C:
1. Create Two Record Types
* “Unqualified” Lead Record Type: Used when Lead Status is not yet “Qualified.” This record type uses a page layout that does not include the Convert button.
* “Qualified” Lead Record Type: Intended only when the Lead is ready for conversion, and it uses a page layout that does include the standard Convert button.

2. Configure Page Layout Assignments
* Assign the “Unqualified” layout to users by default for new leads.
* Assign the “Qualified” layout to users once the Lead moves into a status of “Qualified.”

3. Use Process Builder to Automate Record Type Change
* Create a Process that triggers when the Lead’s Status changes to Qualified.
* The process updates the RecordTypeId to switch the lead to the “Qualified” record type—thereby exposing the Convert button moving forward.