Chemistry questions... Which of the following ionic compound(s) will be more soluble in acid than in water?

Any salt with a basic anion is more soluble in acid than in water. The degree to which this occurs depends on how basic the anion is. (Remember that the weaker the acid is, the stronger the basicity of its anion). Hence, a) and b) are significantly more soluble in acid than in water, because CO3(2-) is the conjugate base of the very weak acid HCO3-, and OH- is the conjugate base of the very weak acid H2O. c) is predicted to be only slightly more soluble in acid than in water, because SO4(2-) is only a weak base (due to HSO4- being a rather strong acid, but not quite "strong"). BaSO4 is however soluble in concentrated sulfuric acid. In d), the anion is the salt of a strong acid (HCl), so adding acid won't protonate it significantly, and the solubility will not increase (in fact, if you add HCl, it will decrease, due to the common ion effect).