Final answer:
To make comments in a document invisible, the correct approach is to apply hidden text formatting. This method ensures the text is invisible under normal conditions and won't be visible unless specifically set to show.
Step-by-step explanation:
To make the comments in a document invisible, you should apply hidden text formatting. Unlike setting the font color to white or using strikethrough formatting, which only visually disguises the text but can still be seen if selected or when printed, hidden text formatting makes the text invisible unless you have the option to show hidden text enabled in your word processor. For completeness, option 'a' will make text invisible against a white background but visible against any other color or if highlighted, option 'b' will not hide the text but rather indicate it is meant to be removed, and option 'c' simply stylizes text without hiding it.