The correct answer is (b), centripetal force.
A centripetal force is a force that acts towards the center of a circle. It is necessary to keep an object moving in a circular path. The centripetal force on a satellite is provided by the gravitational force of the Earth.
The centrifugal force is an apparent force that acts outward on a rotating object. It is not a real force, but rather a result of the inertia of the object. The centrifugal force is not what keeps a satellite in orbit.
The gravitational force is a real force that acts between two objects with mass. The gravitational force between the Earth and a satellite is what keeps the satellite in orbit.
Some other forces, such as the solar wind, can also affect the orbit of a satellite. However, the gravitational force of the Earth is the dominant force that keeps a satellite in orbit.