Final answer:
LDAP (Lightweight Directory Access Protocol) is the most suitable complement to RADIUS for remote access solutions as it manages user access to network resources effectively and securely.
Step-by-step explanation:
The appropriate choice of authentication protocols to complement RADIUS for a company's remote access solution would be LDAP (Lightweight Directory Access Protocol). LDAP is designed to manage user access to a variety of resources within a network, making it a great complement to RADIUS, which provides centralized Authentication, Authorization, and Accounting (AAA) management for computers that connect and use a network service. Whereas protocols like HTTP, SMTP, and SNMP serve different purposes, LDAP specifically interacts with directory services to provide a method for users to access and maintain distributed directory information in an organized, secure manner.