Among common salt sand and copper sulphate , which forms a true solution

                     Common Salt and Copper Sulfate forms a true solution.

Explanation:

                    True Solutions are those solutions in which the solutes are uniformly distributed throughout and forms a homogeneous mixture. In these solutions the solutes either exist in molecular form exist as ions. Like in given cases;

                                 NaCl ₍s₎   →   Na⁺ ₍aq₎  +  Cl⁻ ₍aq₎

and,

                            CuSO₄ ₍s₎   →   Cu²⁺ ₍aq₎  +  SO₄²⁻ ₍aq₎  

Also, the size of solutes in true solutions is less than 1 nm. Also, when allowed for longer time these ions in true solutions never settles down at the bottom. This formation of true solution by these two compounds is mainly due to their greater solubility in water. The ions formed have greater force of interactions with the molecules of eater.

Also, the sand fails to form the true solution because sand in insoluble in water. The force of attractions between the sand and water are smaller than the force of attractions between sand molecules. Thus, when added in water the sand particles precipitates out and settle down at the bottom.

c) should be the answer.