Which one of the following hydride is the thermally least stable

Down the group as the size of the atom increases affinity of hydrogen towards central atom decreases. Thereforea) X-H bond enthalpy decreasesb) Thermal stability decreases….H2O and H2S are exothermic products,remaining three hydrides are endothermic products.c) Reducing power increasesd) acidic strength (Ka) increasese) pKa decreasesDown the group as vanderwaal's forces increasesMelting and boiling points increasesExpected BP and MP….H2O