The longest latitude refers to the line of latitude that spans the greatest distance from east to west on the Earth`s surface. In this case, the correct answer is option 3, the 0.0 degree latitude, also known as the equator.

The equator is an imaginary circle that divides the Earth into the Northern Hemisphere and the Southern Hemisphere. It is located at 0 degrees latitude and runs horizontally across the middle of the Earth. The equator is considered the longest latitude because it spans the greatest distance from east to west. It measures approximately 40,075 kilometers (24,901 miles) in length.

Option 1, the 90 degree latitude, also known as the North Pole or South Pole, is not the longest latitude as it represents a point on the Earth`s surface rather than a line.

Option 2, the 23.5 degree latitude, is known as the Tropic of Cancer or the Tropic of Capricorn, depending on its position in the Northern or Southern Hemisphere. While this latitude is significant, it is not the longest latitude.

Option 4, the 66.5 degree latitude, is known as the Arctic Circle or the Antarctic Circle, depending on its position in the Northern or Southern Hemisphere. While these latitudes