Answer:
(2) Cu2+ + 2e- --> Cu
Explanation:
As per the principle of conservation of charge, for a given equation, the net charge on the reactant side (left side) must be equal to that on the product (right side).
Based on the given options:
1) Cu + e- → Cu+
Left side = -1 ; Right side = +1
Charge is not conserved
2) Cu+ → Cu + e-
Left side = +1, Right side = -1
Charge is not conserved
3) Cu2+ + 2e- → Cu
Left side = (+2) + (-2) = 0; Right side = 0
Charge is conserved
4) Cu2+ → Cu + 2e-
Left side = +2; Right side = -2
Charge is not conserved