What is the approximate inclination of the International Space Station's orbit relative to the equatorial plane of the Earth?

The correct option is 51.6°.

Orbital inclination is defined as the angle:

$i$

between the plane of an object's orbit and the Earth's equatorial plane. An inclination of 0° corresponds to an orbit directly over the equator, whereas an inclination of 90° corresponds to a polar orbit passing over both the North and South poles.

The International Space Station (ISS) operates in Low Earth Orbit (LEO) at an orbital inclination of approximately 51.6° relative to the Earth's equator.

This specific inclination angle was chosen due to key geographical, geopolitical, and orbital mechanics factors:
1. Launch Site Latitude Constraints: When a rocket is launched into orbit, the minimum orbital inclination it can directly reach without executing fuel-expensive orbital plane change maneuvers is equal to the latitude of its launch site.
2. Baikonur Cosmodrome Location: Russia's primary launch site, the Baikonur Cosmodrome, is situated at a latitude of approximately 45.9° N. To avoid dropping rocket stages over populated areas or neighboring countries (such as China) during launch, Russian rockets are launched along a specific flight corridor that results in an inclination of 51.6°.
3. International Accessibility: An inclination of 51.6° allows spacecraft launched from both Baikonur (Russia) and the Kennedy Space Center in Florida, USA (latitude ~28.5° N) to reach the station efficiently without wasting valuable propellant.