What must you do to use the BACKSPACE key to delete a word while in command mode of vi?

To use the BACKSPACE key to delete a word while in command mode of vi, you must switch to insert mode. In vi, command mode is designed for navigating and manipulating text, and it doesn't support direct text editing operations like using the BACKSPACE key to delete text.

When you switch to insert mode, you can type and delete text more intuitively. In insert mode, pressing the BACKSPACE key will remove characters before the cursor, allowing for typical text-editing functionality. Therefore, to achieve the desired action of deleting a word using the BACKSPACE key, transitioning to insert mode is essential.

The other options do not facilitate the described deletion method within the context of vi's operational modes.