Covalent polymeric hydride among the following is

 Due to high ionization energy and small size, Be is always form covalent bonds. ${\mathrm{BeH}}_2$ is an unstable molecule due to incomplete octet so it forms polymer ${\left({\mathrm{BeH}}_2\right)}_n$ . In monomeric form ${\mathrm{BeH}}_2$ formed normal bonds, only four electrons in valence shell of $\mathrm{Be}$ atom and there is an electron deficiency and it is unstable. Therefore, its stable state is in polymeric form each Be atom is bounded by two hydrogen atoms and each hydrogen atom to two Be atoms.