Final answer:
The depth of the potential well is equal to the work function, which is 4.0 eV in this case.
Step-by-step explanation:
The depth (V₀) of the potential well in which the conduction electrons move can be determined by finding the difference between the work function and the energy of the conduction electrons. In this case, the work function of Cu is given as 4.0 eV. Since the work function represents the minimum energy required to remove an electron from the metal, the potential well depth is equal to the work function.
Therefore, the depth of the potential well (V₀) in this case is 4.0 eV. Hence, the correct answer is (a) 4.0 eV.