(A) : Thermal stability of the hydrides of VIA group elements decreases from H₂O to H₂Po.
(R) : The heats of dissociation of M – H bond of hydrides of VIA group decreases down the group.
The correct answer is

$\mathrm{Thermal} \mathrm{stability}\infty \frac{1 }{\mathrm{size} \mathrm{of} \mathrm{atoms}}\infty \mathrm{Bond} \mathrm{dissociation} \mathrm{energy}$

Size of group-16 elements increases down the group, consequently the bond dissociation energy (or heat of dissociation) between M-H bond decreases and hence, the thermal stability decreases.