What is the most secure way to store ISAKMP/IPSec preshared keys in Cisco IOS?

A.
Use the service password-encryption command.

B.
Encrypt the ISAKMP preshared key in secure type 5 format.

C.
Encrypt the ISAKMP preshared key in secure type 7 format.

D.
Encrypt the ISAKMP preshared key in secure type 6 format.