The correct answer is git add update.txt because it stages the specific changes made to update.txt, making it ready for the next commit. The git add . command would stage all changes in the current directory and subdirectories, which is not desired in this scenario if the intention is to only stage update.txt. Using git commit update.txt without the add command would not work because commit attempts to create a new commit from currently staged changes, and it does not accept a file path as an argument. git push update.txt is incorrect because push is used for updating remote refs along with associated objects, not for staging changes.