Which one of the following accurately describes the force of gravity

The force of gravity, also called gravitational force, is the force exerted by the gravity of one object to another object near it. This is dependent on factors like mass of the two objects measured between their centers. 

objective: estimate the error in the vector sum
 Here are a few examples:
                 let a= v+ dv where is the normal value of A & Dv the answer would be the error is a
 let b=v+ dv where is v is the normal value of b and dv is the error in b