To combine multiple files into one output stream, which command is appropriate?

The appropriate command to combine multiple files into one output stream is "paste." This command is specifically designed to merge lines of files. When you use paste, it takes the content of each file and displays them side by side in a single output. This can be particularly useful for comparing data or compiling lists from different files since it maintains the structure of the original data while allowing for simultaneous viewing.

In cases where you want to combine the contents of multiple files into a single line rather than side by side, the "cat" command is often used. However, among the options provided, paste is the most relevant for achieving the goal of combining file outputs in a structured way.

The other commands listed do not serve the purpose of combining files into a single output stream. For instance, "rm" is used for removing files, "tr" is used for translating or deleting characters, and "rmdir" is for removing empty directories. Hence, paste is the clear choice for the specified task of combining multiple files.