The correct answer is option 4: It is the 180° longitude. The International Date Line is an imaginary line that runs from the North Pole to the South Pole, mostly following the 180° longitude. It serves as the dividing line between one calendar day and the next. As you cross the International Date Line from west to east, you add a day to the calendar, while crossing from east to west subtracts a day. This is because as you travel in either direction, you are essentially entering a different time zone. It is important to note that the International Date Line is not a straight line and has been adjusted in some places to accommodate political boundaries and to avoid dividing nations into two different days.