Its A:
To control when the Convert button appears on Leads based on the Lead Status field:

You can use Record Types to create different page layouts.

Based on the Lead Status (e.g., only when status is "Qualified"), assign a different record type with a page layout that includes the Convert button.

Picklist dependencies help ensure the correct options display for dependent fields, but in this scenario, the main control is through record types and page layouts.

Custom buttons, validation rules, or process builders don't control the visibility of the standard Convert button directly.

MR01 The answer is D
Because in B, with validation rules, you can't set the lead status picklist to qualified.

its B and not D, sorry marc but workflows are outadated and impossible to create new ones.

D - https://sfdceverywhere.com/2015/03/16/block-lead-conversion-unless-lead-status-is-qualified/