If two unidentified solids of the same texture and color have different solubilities in 100 grams of water at 20°c, you could conclude that a. they are the same substance. b. they are different substances. c. they have different melting points. d. their solubilities will be the same if the water temperature is increased.

I'm certain it's "D"

...because it can't be "A" or "B" because solubility IS a property but to actually determine whether these two substances are the same or different we would need at least two-three properties (like boiling point or specific heat).

and it can't be "C" because the melting point is just simply irrelevant when comparing the solubility of two substances.