Which of the following force has the longest range?           

The gravitational force has the longest range. Although the gravitational force is the weakest force among the given forces, it can exist between bodies that are millions of kilometers away from each other. Therefore, we say that it's a long-range force.

The nuclear force is a very strong force but it exists between the particles of nuclei which are very closely packed.

The electrostatic force is also not a very long-range force the charges can't attract each other if they are placed very far from each other.

Therefore only option A is correct.