The force of is the force at which the earth attracts another object towards itself.
a. inertia
b. friction
c. gravity
d. momentum

The correct option is C. Gravity, and the complete sentence is: "The force of gravity is the force at which the Earth attracts another object towards itself". In fact, the force of gravity between two objects is given by

where G is the gravitational constant, m1 and m2 the masses of the two objects, r their separation. If we take the Earth as one of the two objects, then m1 represents the Earth's mass, m2 the mass of the object and r the distance between the center of Earth and the object, and F is the gravitational force at which the Earth attracts the object.