Enthalpy of atomisation is the energy required to break a solid metal into isolated gaseous atoms. It depends on metallic bond strength, which is influenced by number of unpaired d-electrons available for bonding.

Among d block metals, zinc (Zn) has the lowest enthalpy of atomisation because it has the electronic configuration $\left[\mathrm{Ar}\right]{3d}^{10}{4s}^2$. Its d-orbitals are completely filled (no unpaired electrons), resulting in weaker metallic bonding compared to others with unpaired electrons.

But it is not transition metal 

So Final answer: Cu