In group 17, 2nd row, element 'A' is F
In group 16, 2nd row, element 'B' is O
In group 15, 2nd row, element 'C' is N
Decreasing order of electronegativity
(EN) = F > O > N
Decreasing order of ionization energy
(IE) = F > N > O
Decreasing order of electron gain enthalpy
(EGE) = F > O > N