Thermal stability of hydrides of first group elements follows the order-
A] $LiH > NaH > KH > RbH$
B] $LiH > KH > NaH > RbH$
C] $LiH > RbH > KH > NaH$
D] $LiH > KH > RbH > NaH$

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Hint: Alkali metals react with dry di-hydrogen at about 673K to form crystalline hydrides which are ionic in nature and have high melting points .
When we move down the group the size of alkali metals increase and nuclear charge decreases due to which the force of attraction between the nucleus and outermost shell decreases. Hence the bond dissociation energy decreases . iot becomes easy to break their bonds as going down the group. They become less stable and hence thermal stability decreases down the group.

Complete step by step answer:
A] $LiH > NaH > KH > RbH$- as the thermal stability on going down the group decreases due to decrease in nuclear charge and increases in size. This option is correct
B] $LiH > KH > NaH > RbH$- as sodium comes first in group than potassium and it has small size and due to small size it will require more temp to dissociate so have high thermal stability than potassium it will be more stable so this option is incorrect.
C] $LiH > RbH > KH > NaH$- this order is not correct as rubidium comes last is group and thermal stability decreases down group due to increase in size force of attraction decreases so becomes less stable.
D] $LiH > KH > RbH > NaH$- sodium will have more thermal stability than potassium so the option is incorrect
Our requires ans is “A” that is $LiH > NaH > KH > RbH$


Note:
The alkali metals also react with halogens and form halides which are ionic in nature. But the halides of lithium are covalent in nature due to the fact that lithium ion has high polarisability capability
The alkali metals have strong tendency to get oxidized so they acts as strong reducing agent
Alkali metals dissolve in liquid ammonia and give deep blue colour solutions which are conducting in nature.