Which one of the following hydride of 17th group has the highest melting point

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 decreasesc) Reducing power increasesd) acidic strength (Ka) increasese) pKa decreases….HF has positive pKa value,others have negative pKa values.Down the group as vanderwaal's forces increasesMelting and boiling points increasesExpected BP and MP….HF