a
\(A)\) Radon is anoble gas or inert gas, it has stable filled shell configuration. \((Xe)4 f^{14}\, 5 d^{10} \,6 s^2\, 6 p^6\)

\(B)\) Nitrogen is \(Gr\). 15thelement. it has half filled \(2 p\) orbital. \((He)\,2 s^2\, 2 p^3 \,(He) \,2 s^2 \,2 p^3\)

\(C)\) Oxygen is \(Gr\), \(16th\) element. \((He) \,2 s^2\, 2 p^4\)

\(D)\) Radium is \(Gr,\) \(2nd\) element. \(( R n) \,7 s^2\)

The attachment of electron is most difficult for Radon.