The correct answer is option 3: Half hour.
The standard time of a country is the time used as a basis for coordinating activities within that country. It is often set to match the time at the country`s capital or a specific reference point. The Greenwich Mean Time (GMT) is the reference time used as a basis for standard time around the world. The difference between the standard time of a country and GMT is usually expressed in multiples of an hour. In this case, the correct option indicates that the standard time of a country may differ from GMT by either half an hour before or half an hour after. This means that some countries have a time offset of GMT+0:30 or GMT-0:30.