“X” is the best reducing agent among the hydrides of 15th group and ‘Y’ is the most volatile 15th group hydride. X & Y respectively are

Reducing power ….    ${\mathrm{NH}}_3<{\mathrm{PH}}_3<{\mathrm{AsH}}_3<{\mathrm{SbH}}_3<{\mathrm{BiH}}_3$…….down the group as X - H bond enthalpy decreases , reducing power increases

Boiling point           ….   ${\mathrm{PH}}_3<{\mathrm{AsH}}_3<{\mathrm{NH}}_3<{\mathrm{SbH}}_3<{\mathrm{BiH}}_3$

Boiling point  $\alpha$  $\frac{1}{\mathrm{volatility}}$