B. As can be seen from the figure, the solubility of AB increases with the increase of temperature, and the solubility of C decreases with the increase of temperature. Therefore, at t3℃, the saturated solution of A, B and C is cooled to t2℃, AB precipitates crystals, and C becomes unsaturated solution, and the solute mass fraction is equal to that before cooling. According to the calculation method of solute mass fraction in saturated solution, it can be seen that the solubility in saturated solution is higher, and the solubility of B>A>C at T2℃ is greater than that of A and C at t3℃, so the order of solute mass fraction in the obtained solution from large to small is B > A > C, which is wrong.
The solubility of C and A is greatly affected by temperature, while the solubility of B is less affected by temperature. Therefore, when the concentrated solution of A contains a small amount of substance B at t3℃, it is correct to purify A by cooling the hot saturated solution.
The solubility of D and B increases with the increase of temperature, so when the saturated solution of B is cooled to t 1℃ at t3℃, crystals will precipitate, which is correct.
So choose B.