The correct answer is D. no article.
The equator is the line that divides the Earth into the Northern and Southern Hemispheres. It is also the only line of latitude that is not a parallel. The equator is 0 degrees latitude, and all other lines of latitude are measured north or south of it.
The equator is the only place on Earth where the sun rises directly overhead at noon on the equinoxes. This is because the Earth’s axis is tilted at an angle of 23.5 degrees. The tilt of the Earth’s axis causes the sun to appear to move across the sky in a north-south direction. At the equator, the sun’s path is directly overhead, so it rises and sets at the same time every day.
The other options are incorrect because they are all articles. Articles are used to specify a particular noun. In this case, there is no particular noun that is being specified, so no article is needed.