The correct answer is A because it is crucial to completely remove all traces of the service account key from the repository and prevent future commits of such sensitive information by adding it to the .gitignore file. B is incorrect as deleting the project does not address the exposure of the key in the Git history. C is incorrect because storing keys in Git, even in private repositories, is a security risk. D is incorrect as Google Cloud Support cannot assist in removing the key from your Git repository.