The correct answer is option 3: International Date Line. The International Date Line is an imaginary line that follows the 180° Meridian. This line is used to designate the change in calendar days. When crossing this line from west to east, you would lose a day, and when crossing from east to west, you would gain a day.

Option 1, the Prime Meridian, is incorrect. The Prime Meridian is the imaginary line that divides the Earth into the Eastern Hemisphere and the Western Hemisphere. It is located at 0° longitude.

Option 2, the Equator, is also incorrect. The Equator is the imaginary line that divides the Earth into the Northern Hemisphere and the Southern Hemisphere. It is located at 0° latitude.

Option 4, the Tropic of Cancer, is also incorrect. The Tropic of Cancer is an imaginary line located at approximately 23.5° North latitude. It marks the northernmost point where the Sun appears directly overhead at noon on the summer solstice.

Therefore, the correct answer is option 3: International Date Line.