After a thorough analysis, it was discovered that a perpetrator compromised a network by gaining
access to the network through a Secure Socket Layer (SSL) Virtual Private Network (VPN) gateway.
The perpetrator guessed a username and brute forced the password to gain access. Which of the
following BEST mitigates this issue?

A.
Implement strong passwords authentication for VPN

B.
Integrate the VPN with centralized credential stores

C.
Implement an Internet Protocol Security (IPSec) client

D.
Use two-factor authentication mechanisms