Which of the given option represents a family of salts ?

A 'family of salts' (per the NCERT chapter) is a group of salts that share a common positive radical (cation) or a common negative radical (anion).

• (A) NaCl, $Na_2SO_4$, $CaSO_4$: NaCl has $Cl^-$ as its negative radical while $Na_2SO_4$ and $CaSO_4$ have $SO_4^{2-}$; NaCl and $Na_2SO_4$ share $Na^+$, but $CaSO_4$ shares neither the cation nor the anion with NaCl — so all three do not form one common family. Incorrect.
• (B) $K_2SO_4$, $Na_2SO_4$, $CaSO_4$: All three have the identical acid radical $SO_4^{2-}$ (potassium sulphate, sodium sulphate, calcium sulphate), so they belong to the sulphate family. Correct.
• (C) $NaNO_3$, $CaCO_3$, $Na_2CO_3$: $NaNO_3$ has $NO_3^-$ while $CaCO_3$ and $Na_2CO_3$ have $CO_3^{2-}$ — the negative radicals are not common to all three, and only two of them share $Na^+$. Incorrect.
• (D) $MgSO_4$, $CuSO_4$, $MgCl_2$: $MgSO_4$ and $CuSO_4$ share $SO_4^{2-}$, but $MgCl_2$ has $Cl^-$ instead; only $MgSO_4$ and $MgCl_2$ share $Mg^{2+}$ — no single radical is common to all three. Incorrect.