Which of the following is a command-line tool used for network diagnostics?

The command-line tool used for network diagnostics is "ping." This utility is essential for testing the reachability of a host on an Internet Protocol (IP) network. By sending Internet Control Message Protocol (ICMP) Echo Request messages to the specified address and waiting for a response, it helps determine whether a network device or server is operational and reachable. Additionally, it provides information regarding packet loss and round-trip time, which are crucial for diagnosing network issues.

In contrast, the other options serve different purposes. Notepad is a text editing application and not related to network diagnostics. The command prompt (cmd) is a command-line interface where various commands can be executed, but it is not a tool by itself for network testing. File Explorer (explorer) allows users to navigate the file system on Windows and isn't involved in network diagnostics either. Thus, "ping" is the correct choice as it is specifically designed for troubleshooting network connectivity and performance.