A charge $ {q_1} $ exerts some force on a second charge $ {q_2} $ . If a third charge $ {q_3} $ is brought near, the force of $ {q_1} $ exerted on $ {q_2} $ :
(A) Decreases.
(B) Increases.
(C) Remains unchanged.
(D) Increases if $ {q_3} $ is of the same sign as $ {q_1} $ and decreases if $ {q_3} $ is of the opposite sign.

Hint :Here, we have to discuss the forces applied by the given charges on each other by using the concept of forces acting on these charges and by the influence of other charges. First the system is in charge of $ {q_1} $ and $ {q_2} $ and then charge $ {q_3} $ is brought nearer to them.

Complete Step By Step Answer:
Let know that the two charges are already in the system i.e. $ {q_1} $ and $ {q_2} $ and the third charge is brought nearer to them is $ {q_3} $ .
Force exerted by the charge $ {q_1} $ on charge $ {q_2} $ will remain the same. On introduction of third charge near the charges $ {q_1} $ and $ {q_2} $ the force exerted on $ {q_2} $ by $ {q_1} $ is denoted by $ {F_1} $ . Let us assume that the force on $ {q_2} $ by $ {q_3} $ is denoted by $ {F_2} $ . Therefore, the net force of system be change on introduction of charge to the given system is given by:
Net force $ = {F_1} + {F_2} $
But the charge $ {q_2} $ will experience force exerted by $ {q_1} $ and will still remain the same which is of $ {F_1} $ .
Here, the force has remained unchanged.
Thus the correct answer is option C.

Note :
We have discussed here the forces acting on all the charges and the forces have changed or remained the same. We have concluded here the charge introduced in two charge systems does not change the force between the previous two charges.