What dots Office 365 use natively for authentication?

Microsoft CHAP (MS-CHAP) is a challenge-handshake protocol designed for dial-up and VPN remote access authentication, not for cloud-based Office 365 service authentication.

Exchange Online is the native Office 365 messaging service that handles mail flow and integrates with the Microsoft 365 authentication stack for mail-based protocols and service access. It works alongside Azure AD to authenticate users against Office 365 mail and collaboration services.

Azure Active Directory (now Microsoft Entra ID) is the cloud-based identity and access management service that Office 365 relies on natively for user sign-in, single sign-on, conditional access, and token-based authentication across all Microsoft 365 applications.

Central Authentication Service (CAS) is an open-source SSO protocol originally developed by Yale University and is not a Microsoft product or native component of Office 365.

Extensible Authentication Protocol (EAP) is a framework for network access authentication in 802.1X and Wi-Fi environments, not for Office 365 cloud application authentication.