Added, though it's worth noting the official name of South Korea is the Republic of Korea as it doesn't recognize the North, hence why I was a bit hesitant to put it at first. It'd be like accepting Taiwan for China. But nonetheless, added.
The official name of Singapore is the Republic of Singapore. Sri Lanka is officially the Democratic Socialist Republic of Sri Lanka. In fact many countries "official" names are not what they are commonly known as. Considering quizzes like this have been created many times it's probably best to go with the commonly used names.