A is incorrect. F and Cl belong to the same group of the periodic table. The electron gain enthalpy usually becomes less negative on moving down a group. However, in this case, the value of the electron gain enthalpy of Cl is more negative than that of F. This is because the atomic size of F is smaller than that of Cl.
B is correct (ionisation energy decreases down the group).
C is incorrect (Electronegativity is a fixed property of an element and is typically measured on scales such as the Pauling scale or the Mulliken scale.).
D is incorrect. The oxides which behave as both acidic and basic oxides are called amphoteric oxides. . Here Al2O3 is amphoteric oxide and NO is neutral oxide.
Hence, A, C and D are incorrect.