D,A,L and Q are 17th group elements of Modern periodic table. The correct trend of thermal stability of their hydrides is HD > HA > HL > HQ. The element ‘A’ in the sequence is

Down the group as the size of the atom increases affinity of hydrogen towards central atom decreases. Thereforea) X-H bond enthalpy decreases….HF>HCl>HBr>HI ;b) Thermal stability decreases…..HF>HCl>HBr>HI ;c) Reducing power increases…..HFHCl>HBr>HI ;Down the group as vanderwaal's forces increasesMelting and boiling points increasesExpected BP and MP….HF