Final answer:
NSLOOKUP is the commonly used command for diagnosing and troubleshooting DNS problems.
Step-by-step explanation:
When diagnosing and troubleshooting DNS (Domain Name System) problems, the commonly used command is NSLOOKUP.
NSLOOKUP is a command-line tool used to query the DNS to retrieve information about domain names and IP addresses. It can be used to troubleshoot DNS problems, check DNS records, and perform DNS lookups. Other commands mentioned in the options, such as IPCONFIG, PING, and TRACERT, are not specifically designed for DNS troubleshooting, but they can be useful in network diagnostics and troubleshooting in general.