Answer: nmap
Explanation:
The best tool to use for determining which services are running on Linux servers in a certain subnet is nmap. Nmap (short for "Network Mapper") is a network scanning and security tool that can be used to discover and map networks, as well as to identify the services that are running on specific servers or devices.
Nmap can be used to scan a subnet and identify the hosts and devices that are connected to it, as well as to determine the services that are running on those devices. This can be useful for identifying potential vulnerabilities or security risks, as well as for understanding the network and its capabilities.
In contrast, the other options listed (ifconfig, telnet, and ping) are not as well-suited to the task of determining which services are running on Linux servers in a subnet. Ifconfig is a command-line utility that is used to configure network interfaces, but it does not provide information about services running on servers. Telnet is a protocol for connecting to remote servers, but it does not provide information about the services that are running on those servers. Finally, ping is a tool for testing network connectivity, but it does not provide information about services running on servers.