1. Elastic collisions: Objects bounce apart after they have collided with each other; Kinetic Energy is CONSERVED so you can use Conservation of Mechanical Energy; Momentum is CONSERVED so you can use Conservation of Linear Momentum.Inelastic collision: Objects stick together/embedded after collision; Kinetic Energy NOT conserved so CANNOT use Conservation of Mechanical Energy; But Momentum is CONSERVED so you can use C.O.L.M

2. For Ball A, it is NEGATIVELY CHARGED but Ball C is Neutral. Neutral means the NET CHARGE IS ZERO (i.e. number of negative charges = number of positive charges). Since C is insulator, the Negatively charged Ball A can induce the Positive charges at the surface of insulator A while repelling Negative charges. It is similar to the diagram shown below.Since the positive charges on the insulator surface C is nearer to Ball A, by Coulombs Law, the attractive force is a bit larger than the repulsive force due to negative charges on the insulator C. Therefore, it is weakly attractive.