Q. which protocol would best serve to authorize users to access directory services?

Lightweight directory access protocol (LDAP)

Explanation:

Lightweight directory access protocol is a protocol that provides a common open protocol for interfacing and querying a directory service information given by a network operating system. LDAP is an application layer protocol that uses port number 389 via tcp and udp. LDAP queries are transmitted in text and allows for all or some data be queried anonymously.

Lightweight directory access protocol, ordinarily does not require user authentication connection, but this can be configured to ask for user credentials.