Both A and D are in widespread use. Neither is better than the other, and if the test only accepts D, then it's wrong. Sentence B is one I can easily imagine myself saying sometimes, but I'd be less likely to write it. However, I wouldn't consider it wrong, even though I prefer A or D. Sentence C is uncommon, but it could be used when you want to emphasize the word immediately. So I wouldn't say it's wrong, either.
In addition, most sentences omit the before government, even though we need an article here.
The upshot of this is that it appears that this presumed test question was written by someone who isn't a native-level English speaker and who is relying on some set of rules they heard of rather than how people actually use English. I wouldn't rely on their ability to teach English at the level of this question. Remember that correct English is determined by how a large segment of the population uses the language in a given context, and quality learning materials will reflect this.
Upon rereading the question, I note that option D can be interpreted in two ways, whereas the others can't. Options A-C mean that the government took immediate action. Option D can be interpreted that way, but it can also be interpreted slightly differently as the government's action was immediately after the tragedy. However, the difference in meaning is so slight that I doubt it would make any real-world difference.