Which of the following is the default authentication method for a Windows client using PPP over a dialup connection?

TACACS+ is a Cisco-proprietary AAA protocol used for device administration authentication, not for Windows PPP dialup sessions.

WINS (Windows Internet Name Service) is a NetBIOS name resolution service and has no role in PPP authentication.

Kerberos is a ticket-based authentication protocol used for domain authentication in Active Directory environments, not for PPP dialup connections.

MS-CHAP (Microsoft Challenge Handshake Authentication Protocol) was specifically developed by Microsoft as the default PPP authentication protocol for Windows dialup clients. It uses a three-way handshake with a challenge and hashed response to authenticate without sending the password in cleartext. MS-CHAPv2 extended this with mutual authentication and is still referenced in Windows PPP and VPN configurations.