What command is used in Kerberos to determine if a ticket is valid?

The command used in Kerberos to determine if a ticket is valid is 'klist.' This command is specifically designed to display a list of Kerberos tickets held in a user's credential cache, including information about each ticket, such as their validity period, the principal associated with the ticket, and the realm it corresponds to.

In the context of Kerberos authentication, checking the validity of a ticket is crucial, as it allows users to verify that their credentials are active and that they can access resources that require authentication.

While 'kinit' is used to obtain a Kerberos ticket and create a new session, it does not provide information about whether an existing ticket is valid. The other options, 'telnet' and 'openssh,' are network protocols and tools related to remote access and do not relate to the functionality of managing Kerberos tickets. Therefore, 'klist' is the correct choice for determining the validity of a Kerberos ticket.