The correct answer is option 3, Equator. The Equator is known as the Great Circle because it is the largest circle that can be drawn on the Earth`s surface. A Great Circle is defined as any circle on the Earth`s surface whose center is the center of the Earth. It divides the Earth into two equal halves, the Northern Hemisphere and the Southern Hemisphere.

Option 1, Tropic of Cancer, is not the correct answer. The Tropic of Cancer is a latitude line located at around 23.5 degrees north of the Equator, marking the farthest point north where the sun can appear directly overhead.

Option 2, Tropic of Capricorn, is also not the correct answer. The Tropic of Capricorn is a latitude line located at around 23.5 degrees south of the Equator, marking the farthest point south where the sun can appear directly overhead.

Option 4, Arctic Circle, is not the correct answer either. The Arctic Circle is a latitude line located at around 66.5 degrees north of the Equator, marking the boundary of the Arctic region.

In summary, the Equator is known as the Great Circle because it is the largest circle that can be drawn on the Earth`s surface and