Which of the following is characterized by existing at a discrete energy level?

Electrons always have discrete energy levels, no matter whether they are in isolated atoms, in a multi-atom molecule, or in an extended lattice of atoms in a solid. The difference among these cases are the sizes of the energy differences separating the different energy levels available to the electrons in the system. 

The activity series is shown in the attached picture. if the element is below the other, then you can displace the other. following the activity series as a guide, the reactions are as follows: nickel and aluminum chloride3 ni + 2 alcl₃  → 3 nicl₂ + 2 albarium and copper bromideba + cubr  →  no reaction because cu is less oxidizable than bapotassium nitrate and aluminum3 kno₃ + al  → al(no₃)₃ + 3 ksodium phosphate and zincnaso₄ + zn  → znso₄ + na