The imaginary line on the Earth's surface which closely follows 180° Meridian…

Lines of longitude (meridians) run from pole to pole and mark a location's position east or west of a reference meridian, while lines of latitude run parallel to the Equator and mark position north or south. Longitude spans 360° for a full 24-hour day, so every 15° of longitude corresponds to one hour; the meridian exactly 180° from the Prime Meridian therefore marks a 12-hour offset from Greenwich Mean Time, and by international convention it is where the calendar date changes.

Applying this to the item: the imaginary line on the Earth's surface that closely follows the 180° meridian — and where the calendar date advances or falls back by one day on crossing it — is called the International Date Line.

Why the other lines do not fit:

• Tropic of Cancer — a line of latitude at 23.5°N marking the sun's northernmost overhead position at the June solstice; it runs parallel to the Equator and has no connection to the 180° meridian or the date-change convention.

• Equator — the 0° line of latitude dividing the Earth into Northern and Southern Hemispheres; also a latitude line, unrelated to longitude-based date reckoning.

• Prime Meridian — the reference line of longitude at 0°, passing through Greenwich, from which all other meridians (including the 180° one) are measured; it is the starting point of longitude, not the 180° line itself, and carries no date-change convention.

Cross-check: reapplying the definition — the meridian at 180° from Greenwich, where the calendar date changes, is by definition the International Date Line, matching the value identified above.