Final answer:
Kerberos is the default authentication type for Active Directory, offering a higher level of security with mutual authentication compared to earlier protocols such as NTLM.
Step-by-step explanation:
The default authentication type for Active Directory is Kerberos. Initially developed by MIT as a part of Project Athena, Kerberos is a network authentication protocol designed to provide strong authentication for client/server applications by using secret-key cryptography. Active Directory employs Kerberos as the default authentication method starting with Windows 2000 and onwards, which offers mutual authentication — both the user and the service verify each other's identity. Kerberos is considered to provide a higher level of security when compared to the earlier default, NTLM (NT LAN Manager).