Which hydride among the following is less stable?

Among the following,the correct statements are:
$I$. $LiH, BeH_2$ and $MgH_2$ are saline hydrides with significant covalent character.
$II$. Saline hydrides are volatile.
$III$. Electron-precise hydrides are Lewis bases.
$IV$. The formula for chromium hydride is $CrH$.
The correct option is:

The sum of the number of neutrons and protons in the radioisotope of hydrogen is

Deuterium resembles hydrogen in properties but:

Identify the catalyst used in the following reaction: $CO_{(g)} + H_2O_{(g)} \xrightarrow{623 \ K} CO_{2_{(g)}} + H_{2_{(g)}}$

In all its properties,hydrogen resembles