According to the FortiGate Administration Guide, "EAP-TLS is the most secure EAP method. It requires a digital certificate on both the server end and the client end. The server and client authenticate each other using their certificates." Therefore, option D is true because it describes the EAP method that requires the use of a digital certificate on both the server end and the client end. Option A is false because EAP-TTLS only requires a digital certificate on the server end, not the client end. Option B is false because PEAP also only requires a digital certificate on the server end, not the client end. Option C is false because EAP-GTC does not require a digital certificate on either the server end or the client end.