Which of the following hydrides is electron deficient?

An alkali metal hydride $(NaH)$ reacts with diborane in '$A$' to give a tetrahedral compound '$B$' which is extensively used as a reducing agent in organic synthesis. The compounds '$A$' and '$B$' respectively are

Identify the correct statements about the hydrides.
$(I)$ In saline hydrides,the oxidation state of hydrogen is $-1$.
$(II)$ $LaH_{2.87}$ is an example of an interstitial hydride.
$(III)$ $NH_3$ and $H_2O$ have the tendency to form hydrogen bonds.
$(IV)$ Electrolysis of molten sodium hydride liberates $H_2$ gas at the anode.

Spin isomerism is shown by

What do you understand by the term non-stoichiometric hydrides? Do you expect this type of hydrides to be formed by alkali metals? Justify your answer.

Hydrogen accepts an electron to form an inert gas configuration. In this property,it resembles: