A. Add a rich text area to the Case Lighting page > Set the component visibility of the rich text area to show when the account support level is platinum.

B. Create a text-only Visualforce page > Drag the Visualforce component into the Case page layout > Set its visibility to show when the account support level is platinum.

C. Create a text-only Visualforce page > Clone the case page layout > Drag the Visualforce component into the page, and assign the layout to platinum cases.

D. Clone the Case Lightning page > Add a rich text area to the new page, and assign this page to platinum accounts.