C. Find and replace
This command is often found under the "edit" menu of many programs. On the toolbar, the undo button is a backwards (sometimes curved) arrow. More specifically, the arrow points to the left as if to mean "go back and fix". Likely it points to the left due to many western languages reading from left to right, so the undo arrow goes in the opposite direction.
In contrast, the "redo" button points in the other direction to repeat the last action you applied. The redo button also undoes whatever the undo button did. For instance, if you deleted a word and then hit undo, then the word would come back. Hitting redo will delete the word again.