Which of the following diagrams best depcits an alloy of $\ce{Ni}$ and $\ce{B}$ ?

An alloy is a substance that contains a mixture of elements and has metallic properties. Nickel is a metal but boron is not. We should therefore expect the formation of an interstitial alloy. In an interstitial alloy, the holes in the metal structure are filled by smaller atoms. Boron is considerably smaller than nickel, being one period above on the periodic table.