Identify the Windows-based command used by Ethan to view information about all opened sessions.

The command used to view information about all opened sessions in a Windows environment is "net session." This command provides details regarding active connections to a shared resource on the system, allowing an administrator to see who is connected and their session status.

The other mentioned options serve different purposes. "Event ID 4657" pertains specifically to Windows Security Event Logs, indicating changes to objects like files or registry keys, rather than active sessions. "Get-EventLog" is a PowerShell command used to retrieve the events from event logs on a local or remote machine but doesn't focus specifically on user sessions. "Whoami" displays the current user’s identity and context but does not provide information about all open sessions on the machine.

In summary, "net session" is the command that specifically meets the requirement of viewing information about opened sessions, making it the correct choice.