The correct answer is The Equator.

• A Great Circle is any circle that circumnavigates the Earth and passes through the center of the Earth.
• A great circle always divides the Earth in half, thus the Equator is a great circle (but no other latitudes) and all lines of longitude are great circles. 
• The Equator:
  • It is an imaginary line on the surface of the Earth at the same distance from the North Pole and the South Pole.
  • It divides the Earth into the Northern and Southern Hemispheres.
  • On the northern side of this line are the Tropic of Cancer at 23½ ° and the Tropic of Capricorn at 23½ ° on the southern side.
  • The equator passes through the land or waters of 14 countries.​

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• The Tropic of Capricorn:
  • It is an imaginary line drawn from the west to the east, at 23° 23 '22" parallel to the equator in the southern hemisphere.
  • The area to the north of the Tropic of Capricorn and south of the Tropic of Cancer is called the "tropical region".​
• The Tropic of Cancer:
  • It is a latitude line drawn from west to east at 23.5 degrees in the Northern Hemisphere, parallel to the equator on the globe.
  • It is the northernmost latitude line of the Earth, on which the sun shines vertically in the afternoon.
• The Arctic Circle:
  • The position of the Arctic Circle is 66° 33′46.6 ″ north of the equator.
  • Its latitude depends on the axial tilt of the Earth