What does Word use to mark words it doesn't recognize?

Word uses a red wavy underline to mark words it doesn't recognize, primarily signifying spelling errors. This visual cue is important for users as it helps identify potentially misspelled words in the document being edited. For instance, if a word is typed that isn't found in the spelling dictionary, it gets that distinctive red underline, prompting the user to review the word for accuracy or to see if it should be added to the dictionary.

In comparison, other colors, like the blue wavy underline, are used for grammatical issues, green for stylistic suggestions, and yellow may indicate certain types of annotations or comments, but the standard method for highlighting unrecognized spelling is the red wavy underline. Thus, the choice of red is specifically designed to draw attention to spelling mistakes, making it the correct answer to the question.