B, because the solubility of B increases with the increase of temperature, the saturated solution of B with a certain mass at t 1℃ will become unsaturated when the temperature rises at t2℃, so the option is correctly expressed;
C, according to the solubility curves of the three substances, the solubility relationship of the three substances is B > C > A at t 1℃, that is, the mass fraction of solute in the saturated solution of the three substances is b > c > a at t 1℃; Because the solubility of A and B increases with the increase of temperature, the solubility of C decreases with the increase of temperature. When their saturated solution is heated to t2℃, the saturated solution of A and B will become unsaturated solution, and the mass fraction of solute in the solution will remain unchanged, and crystals will precipitate in the saturated solution of C, and the mass fraction of solute in the solution will decrease. Because the solubility of C at t2℃ is less than that of A at t 1℃, the mass fraction of solute in the solution after heating is B > A > C, so the option is correctly expressed.
D, according to the option analysis of C, the mass fraction of solute in solution B is the largest at t2℃, so the option expression is wrong;
So choose D.