Which is polymeric hydride?

$\begin{array}{1 1}(a)\;CaH_2\\(b)\;MgH_2\\(c)\;BaH_2\\(d)\;SrH_2\end{array}$

The hydrides of beryllium and magnesium are covalent and polymeric. it has a chain structure containing chains with hydrogen bridges
$MgH_2$ is polymeric hydride.
Hence (b) is the correct answer.
edited Nov 18, 2017