(a) BeH2 and MgH2 are polymeric hydrides, a large number of BeH2 and MgH2 units join together by hydrogen bonding to produce a long chain. (b) LiH is by no means unreactive towards air at moderate temperature because it is a powerful reducing agent. (c) BeH2 and MgH2 are covalent in nature due to small size of Mg2+ and Be2+ ions and H−ions is large anion. (d) The statement (d )is incorrect. The correct stability order of alkali hydrides is LiH>NaH>KH>RbH>CsH Hence, (a), (b), (c) are correct statements.