90 degree south latitude marks the South pole.

The South Pole is the southernmost point on the Earth's surface and is located at a latitude of 90 degrees south. It is the opposite end of the Earth from the North Pole, which is located at 90 degrees north latitude.

Latitude and longitude
Latitude and longitude are geographical coordinates that are used to specify locations on the Earth's surface. Latitude measures the distance north or south of the Equator, while longitude measures the distance east or west of the Prime Meridian, which runs through Greenwich, London.

The Equator
The Equator is an imaginary line that divides the Earth into the Northern Hemisphere and the Southern Hemisphere. It is located at 0 degrees latitude and is the starting point for measuring latitude. Locations north of the Equator have positive latitudes, while locations south of the Equator have negative latitudes.

North Pole and Tropic of Cancer
The North Pole is located at 90 degrees north latitude, which means it is the furthest point north on the Earth's surface. On the other hand, the Tropic of Cancer is located at approximately 23.5 degrees north latitude, and it is the northernmost latitude at which the Sun can be directly overhead at noon.

Explanation of the answer
Given that 90 degrees south latitude is mentioned, it means we are looking at a point that is 90 degrees south of the Equator. The only location that fits this description is the South Pole, as it is the only point on Earth that has a latitude of 90 degrees south. Therefore, the correct answer is option A - South Pole.

In summary, 90 degrees south latitude marks the South Pole, which is the southernmost point on the Earth's surface. This point is located at a latitude of 90 degrees south and is opposite to the North Pole, which is located at 90 degrees north latitude.