Final answer:
The appropriate internal source to store the service accounts for the network admin's purpose would be Active Directory (AD). LDAP and flat file/SQL database may not be suitable for user management and access control on this scale.
Step-by-step explanation:
Considering the given scenario, the appropriate internal source to store the service accounts for the network admin's purpose would be Active Directory (AD). AD is a directory service provided by Microsoft and is commonly used in enterprise environments to store and manage user accounts, group policy settings, and network resources. It offers a centralized location for managing user authentication and access control.
LDAP, or Lightweight Directory Access Protocol, is a protocol used to access and authenticate against directory services like AD. While LDAP is a valid option to store user accounts, it is often used alongside AD as the underlying technology.
In this case, a flat file or SQL database may not be suitable for storing the user accounts because they are not designed for user management and access control on such a scale.