Answer :
The DNS server is malfunctioning if a user can ping an IP address but not the host name, as it's responsible for translating host names into IP addresses. Therefore, the correct answer is : (a).
If a user can ping the IP address of a web server but cannot ping the web server host name, the network server that is malfunctioning is the DNS server. When a user attempts to reach a website by its host name (such as www.example.com), the DNS server is responsible for translating that host name into an IP address that the network can understand. If the DNS server is not functioning correctly, the translation process fails, and while the user might still reach the server using its IP address, the host name remains unresolved.