Which of the following identification and authentication factors are often well known or easily discovered by others on the same network or system?

The identification and authentication factor that is often well known or easily discovered by others on the same network or system is the username. Usernames are typically not secret and are often publicly visible, especially in environments like corporate intranet applications, email systems, and social media. Since usernames can be easily observed or even guessed, they do not provide a strong level of security on their own.

In contrast, while passwords are also widely used for authentication, they are intended to be kept secret and are usually not publicly visible unless shared inappropriately. Biometric reference profiles, which are based on unique physical characteristics, are inherently private and cannot be easily discovered by others. Similarly, a PGP (Pretty Good Privacy) secret key is designed to be confidential and is not something that can be easily identified or accessed by others without proper authorization. Thus, the username stands out as a factor that is more easily known or compromised within a given network environment.