Statement (A): Sodium exhibits variable valency

 Statement (B): Nitrogen variable valency are 2 and 4. 

Statement (C): Gold exhibits variable valency.

Sodium, Na is an alkali metal and it exhibits mono valence of its compounds. Nitrogen is a VA group element. It shows variable valences 3 and 5.

Gold (Au) is a transition metal; it exhibits variable valency 1 and 3.

$\mathrm{Ex}: \mathrm{AuCl} \& {\mathrm{AuCl}}_3$

Hence, the given three statements A, B are incorrect and C is correct.