Which authentication method should Dr Billchoose?

Dr Billis preparing to implement remote-access VPNs, using VPN-1/FireWall-1 and SecureClient.
When Dr Billselects an authentication method, it must meet the following requirements:
The authentication method must support existing authentication methods, including OS passwords
and RADIUS, for ClientAuthentication.The Enforcement Module must use certificates, to
authenticate itself to the client.The authentication method must be flexible, allowing other
authentication solutions to be added, including SecureID and TACACS.

Which authentication method should Dr Billchoose?

A.
Digital Certificates

B.
Pre-shared Secrets

C.
LDAP

D.
Public Key Signatures

E.
Hybrid Mode