The correct answer is option 4, the Equator.

The Equator is the longest parallel of latitude because it is located at 0 degrees latitude and divides the Earth into the Northern Hemisphere and the Southern Hemisphere. It is exactly halfway between the North Pole and the South Pole.

Option 1, the Tropic of Cancer, is located at approximately 23.5 degrees north latitude. It marks the northernmost point where the Sun can be directly overhead at noon on the summer solstice.

Option 2, the Tropic of Capricorn, is located at approximately 23.5 degrees south latitude. It marks the southernmost point where the Sun can be directly overhead at noon on the summer solstice.

Option 3, the Arctic Circle, is located at approximately 66.5 degrees north latitude. It marks the southernmost latitude where the Sun can stay above the horizon for 24 hours during the summer solstice.

Therefore, the Equator is the correct answer as it is the longest parallel of latitude, dividing the Earth into the Northern and Southern Hemispheres.