Which of these authentication combinations would provide the highest level of security for an IPsec remote-access VPN client?

Which of these authentication combinations would provide the highest level of security for an IPsec remote-access VPN client?

A.
pre-shared key and xauth (RADIUS server)

B.
certificate and xauth (local server)

C.
certificate and xauth (RSA SecurID token)

D.
pre-shared key and xauth (RSA SecurID token)

E.
pre-shared key and xauth (local server)

F.
certificate and xauth (RADIUS server)