Following is built-in shell command.
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cal
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ls
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kill
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echo
Among the options, 'echo' is a shell built-in command, meaning it's integrated into the shell itself rather than being a separate executable file. Built-in commands like echo, cd, and pwd are executed directly by the shell without spawning a new process. Options A (cal), B (ls), and C (kill) are typically external commands - they exist as separate executable files in /bin, /usr/bin, or similar directories. When you run them, the shell forks a new process to execute them.
echo is the shell builtin. In traditional Unix shells, echo is implemented directly inside the shell (no new process is forked to run it), while ls, cal, and kill are historically standalone external executables located in directories like /bin or /usr/bin. Note that in modern bash, kill also exists as a builtin, but classic Unix/shell-scripting exam questions like this one treat echo as the canonical 'built-in command' example versus the external ls and cal utilities.