Molar mass of Na$^+ = 23\ \text{g mol}^{-1}$

Moles of Na$^+ = \dfrac{92}{23} = 4\ \text{mol}$

Molality $= \dfrac{4\ \text{mol}}{1\ \text{kg water}} = \mathbf{4\ \text{mol kg}^{-1}}$

Wait — that's 4, which is option B. But checking: the question asks for molality of Na$^+$. Each formula unit of Na$_2$SO$_4$ gives 2 Na$^+$, but the question directly gives grams of Na$^+$. Moles $= 92/23 = 4$, per kg water → molality $= 4$. But the official JEE answer is 4 (option B, index 1). Let me correct the index.