Explanation
According to the documentation, not-applicable means that the Palo Alto device has received data that will be discarded because the port or service that the traffic is coming in on is not allowed, or there is no rule or policy allowing that port or service. This occurs because the traffic was dropped or denied before the application match could be performed. References: 1 Not-applicable in Traffic Logs - Palo Alto Networks 2 Not-Applicable, Incomplete, Insufficient Data in the Application Field - Palo Alto Networks
https://knowledgebase.paloaltonetworks.com/KCSArticleDetail?id=kA10g000000ClspCAC

Explanation
According to the Palo Alto Networks documentation1, the firewall can use three external authentication services to authenticate admins into the Palo Alto Networks NGFW without creating administrator accounts on the firewall: RADIUS, TACACS+, and SAML. These services allow the firewall to verify the credentials of admins against an external server and grant them access based on their assigned roles and permissions.
Therefore, the correct answer is A, B, and E.
The other options are not external authentication services that the firewall can use to authenticate admins:
Kerberos: This option is not an external authentication service that the firewall can use to authenticate admins. Kerberos is a protocol that allows users to access network resources using a single sign-on mechanism. The firewall can use Kerberos to authenticate users for GlobalProtect VPN or Captive Portal, but not for admin access2.
LDAP: This option is not an external authentication service that the firewall can use to authenticate admins. LDAP is a protocol that allows querying and modifying directory services over a network. The firewall can use LDAP to retrieve user and group information from an external server, but not to authenticate admins3.
References: 1:
https://docs.paloaltonetworks.com/pan-os/9-1/pan-os-admin/authentication/authentication-types/external-authent
2:
https://docs.paloaltonetworks.com/pan-os/9-1/pan-os-admin/authentication/authentication-types/kerberos-authen
3:
https://docs.paloaltonetworks.com/pan-os/9-1/pan-os-admin/user-id/map-ip-addresses-to-users/map-ip-addresses

b) Explicit proxy allows for easier troubleshooting, since the client browser is aware of the existence of the proxy12. This means that the client can see the proxy's IP address and port number, and can use tools like ping or traceroute to check connectivity and latency issues. Transparent proxies are invisible to the client browser, which makes it harder to diagnose problems.
c) Explicit proxy supports interception of traffic using non-standard HTTPS ports3. This means that the proxy can handle HTTPS requests that use ports other than 443, which may be required by some applications or websites. Transparent proxies can only intercept HTTPS traffic on port 443, which limits their functionality.

Explanation
https://knowledgebase.paloaltonetworks.com/KCSArticleDetail?id=kA10g000000ClEiCAK