Which of the following statements are not correct? A. The electron...

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 ${Al}_{2}{O}_{3}$ is amphoteric oxide and $NO$ is neutral oxide.

Hence, A, C and D are incorrect.