Satellites orbiting the earth have finite life and sometimes debris of satellites fall to the earth. This is because

The correct option is: of viscous forces causing the speed of satellite and hence height to gradually decrease.

Detailed Explanation:
Although the space where satellites orbit the Earth is extremely thin, it is not a perfect vacuum. In the upper regions of the Earth's atmosphere (such as the thermosphere where many Low Earth Orbit or LEO satellites reside), there are still trace amounts of gas molecules and particles.

As a satellite moves through this dilute atmosphere at very high speeds, it experiences a small but continuous resistive force due to air resistance or atmospheric drag. This drag behaves as a viscous force acting opposite to the direction of the satellite's velocity.

The continuous work done against this viscous force dissipates the mechanical energy of the satellite. As the total energy $E=-\frac{GMm}{2r}$ of the orbit decreases (becomes more negative), the orbital radius $r$ must decrease, causing the satellite's height above the Earth to gradually decay.

Eventually, as the altitude decreases, the atmospheric density increases, which further accelerates the drag. This spiral trajectory downwards causes the satellite to eventually re-enter the denser parts of the Earth's atmosphere, where most of it burns up due to intense frictional heating, and any surviving debris falls to the Earth.