The International Date Line broadly follows which meridian?

The correct answer is C, the 180 degree meridian, which lies in the Pacific Ocean opposite Greenwich. The line does not follow it exactly: it bends east and west to keep the eastern tip of Siberia, the Aleutian Islands and several Pacific island groups on one side, so that no country is left with two dates at the same moment.

Option A, the zero degree meridian, is the Prime Meridian at Greenwich, the starting line for longitude, not for dates. Option B, 90 degrees east, and option D, 90 degrees west, are ordinary meridians with no special role. The rule to remember for the crossing is that a traveller going from east to west across the line adds a day, while one going from west to east repeats the day just passed. This is why a voyage westward around the world appears to lose a calendar date unless the line is accounted for.