* NTP (Network Time Protocol) is used for clock synchronization between computer systems over packet-switched, variable-latency data networks. In the context of security features available when implementing NTP, symmetric server passwords (Option A) and encrypted authentication mechanism (Option D) are correct. Symmetric server passwords involve the use of shared secret keys that are configured on both the client and server to authenticate time exchange. Encrypted authentication mechanisms ensure that the data is encrypted during transit to protect it from unauthorized access or tampering. * References: The information can be referenced from Cisco's official documentation on Implementing and Operating Cisco Service Provider Network Core Technologies.