all the charged particle have the same poarity
the equilibrium is unstable
all the charged particles cannot have the same polarity
both (b) and (c) are correct.
If a charged particle is in equilibrium under electrostatic force, then that particle never be in stable equilibrium. Hence option B is correct. Since all charges are in equilibrium, net force an any one of the charges are in equilibrium, so net force an any one of charges should be zero - for this two equal and opposite forces will act on any charge. Hence option C is correct.