Assertion(A): The basic nature of VA group (Nitrogen family) hydrides decreases from nitrogen to bismuthine.
Reason(R): Availability of lone pair on central atom in hydride decreases down the group.
A.Both (A) an (R) are true and (R) is the correct explanation of (A).
B.Both (A) an (R) are true and (R) is not the correct explanation of (A).
C.(A) is true and (R) is false
D.(A) is false and (R) is true

Hint: Hydrides of VA group behave as Lewis bases. Lewis bases are chemical compounds which can donate lone pairs to other electron deficient compounds which is Lewis acid. Strength of Lewis bases depends upon the capacity of a molecule to donate its lone pair.

Complete step by step answer:
Hydrides of nitrogen families are Lewis bases because they have lone pairs of electrons and they can donate their lone pair to Lewis acid.
Strength of Lewis base is dependent on its capacity to donate lone pairs of electrons or availability of lone pairs.
Availability of lone pairs mainly depends on electron density of an atom. Here nitrogen has the smallest size due to which it has greatest electron density in its group and size of bismuth is large and also has vacant d and f orbital due to which availability of lone pair is less and it is a weak base.
Also, when we move down in a group, atoms have vacant d-orbit due to which they can take lone pairs from other atoms and behave as Lewis acid due to which their basicity decreases.
Hence, Basicity of the hydride nitrogen family decreases due to decrease in availability of lone pairs. Then, assertion (A) and reason (R) both are true and (R) is the correct explanation of (A).
Hence the correct answer is option A.

Note:All hydrides other than nitrogen hydrides can behave as Lewis acid because they have vacant orbital and nitrogen doesn’t have vacant d-orbital. When these hydrides react with halo-acids they donate their lone pair to halogen and make coordinate bones. For example:
$\ddot N{H_3} + HCl \to N{H_4}Cl\left( {N{H_4}^ + + C{l^ - }} \right)$ .