Normal Subgroups and Quotient Groups · Medium problem

Exactly one of these four maps $\varphi_a: \mathbb Z_{12} \to \mathbb Z_8$, $k \mapsto ak \bmod 8$, fails to be a well-defined homomorphism (because $\varphi_a(12) \ne \varphi_a(0)$). Which value of $a$ is it?