Answer:
Correct answer is d (d. both b and c)
Explanation:
Option a cannot be correct answer because frictional force does not act on planets which are moving in their orbit.
Gravity and inertia is correct answer because due to gravity planet does not move out of its orbit and due to inertia (according to law of inertia or Newton's first law if a body is in motion it will continue in motion or if it is at rest it will continue at rest until and unless an external force is applied on it) planet continuously moves in its orbit without moving in any other direction.
