Statement (A):Sodium exhibits variable valency

Statement (B):Nitrogen variable valencies 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 valencies 1 and 3.

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

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