Lewis base strength is maximum for

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….NH3 is exothermic product,remaining four 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….NH3