Final answer:
The Redo function is the option that enables you to keep the last grammatical change after it has been undone. The correct answer is option A.
Step-by-step explanation:
The option that enables you to keep the last grammatical change in a document is the Redo function. The Undo function is used to reverse the most recent change you have made. However, if you use Undo and then realize you needed that change after all, the Redo option is there to reapply it.
In contrast, the Repeat function can be used to apply the last action again to a different selection or in a new location. Finally, Preserve is not typically a function found in text editors or word processing applications related to changes and edits.
To summarize the functionalities, Undo takes you one step back, Redo moves you forward after an undo action, Repeat applies the last action again while Preserve doesn't relate directly to the action of making or keeping changes. Therefore, the correct answer to the question is option A) Redo.