Ctrl + Spacebar in Microsoft Word removes all character-level formatting from the selected text, reverting it to the default font style of the document. It clears bold, italic, underline, color changes, and other font effects while preserving the text itself. This is different from Ctrl + Q which removes paragraph formatting. Options A and B are incorrect because Ctrl + Spacebar does not apply heading styles. Option D is incorrect because it does not change font size specifically.