C
Explanation:
The force of gravity is proportional to the amount of mass in the system, and is inversely proportional to the separation distance between the two masses.
Adding mass to one of the objects would increase the gravitational field it produces, which in return increases the force of gravity.
Increasing the charge of one of the objects would technically add mass to the system, but it can be considered negligible. The adding of charge would cause an attraction between the two objects, but that is not the gravitational force. It is the Coulomb force.
Reducing the mass of one of the objects would decrease the gravitational field it produces, which would cause the force of gravity to decrease. This is the answer.
Moving the objects closer would cause the denominator of the force equation to decrease. A smaller denominator would cause the equality to increase, aka the gravitational force.