Which of the Half-Reactions Below Would Be Complementary and Balance

Which of the half-reactions below would be complementary and balance the following half-reaction? Zn+2 → Zn

A) Cu → Cu+2
B) K+ → K
C) 2 Cu → 2 Cu+2
D) 2 K+ → 2 K
E) Cl2 → 2 Cl-