A penetration tester is attempting to perform reconnaissance on a customer's external-facing footprint and reviews...

The presence of specific services like SSH, NTP, Rsync, and LDAP servers is indicative of a Unix-like operating system. Among the given options, FreeBSD is the most likely operating system that would be running all these services. FreeBSD is known for its robustness and extensive use in environments requiring stable and secure networking services.

Given the context of penetration testing and the enumeration of these services, FreeBSD's configuration and service management fit well with the identified services. Other operating systems listed (Mac OS X, Microsoft Windows, Linux) might not typically run all these services in a similar configuration, particularly NTP and Rsync, which are more common in Unix-like systems.

References:

FreeBSD documentation on NTP and Rsync: FreeBSD Handbook, FreeBSD Rsync

Enumeration examples from HTB writeups such as Gobox and Writeup which often lead to identifying specific OS based on running services​​​​.