Your answer is C. Dark Matter.
Gravitational force is given by,
Where, m and M are the masses of the objects, R is the distance between them and G gravitational constant.
Gravitational force of the star on planet 1,
Gravitational force of the star on planet 2,
Ratio,
Therefore, the gravitational force of the star on the planet 1 is three times that on planet 2.
because positive and negative attract
Newton's first law states that every object will remain at rest or in uniform motion in a straight line unless compelled to change its state by the action of an external force. This is normally taken as the definition of inertia