The correct answer is option 3 - Equator.

Geostationary satellites are placed in an orbit around the Earth called the geostationary orbit. This orbit is located directly above the equator. In order for a satellite to stay over the same spot on Earth, it needs to be positioned at a specific height and velocity in this orbit. The equator is the ideal location for geostationary satellites because it allows them to effectively cover a large portion of the Earth`s surface.

Option 1 - Tropic of Cancer, and option 4 - Tropic of Capricorn, are both latitudes located at around 23.5 degrees north and south of the equator respectively. Placing a satellite directly above these points would not enable it to stay in a fixed position over the Earth.

Option 2 - either North or South Pole, is incorrect because the geostationary orbit is located around the equator and not near the poles. Satellites in polar orbits are used for different purposes, such as weather monitoring or observation.

Overall, a geostationary satellite needs to be directly above the equator to stay over the same spot on the Earth, making option 3 the correct answer.