Final answer:
The 3.0 M hydrochloric acid (HCl) solution is more concentrated than a 2.0 M solution because molarity signifies the amount of solute per volume of solution.
Step-by-step explanation:
If a 3.0 molar solution of Hydrochloric Acid (HCl) has more molecules of HCl per milliliter of water than a 2.0 molar solution, this means that the 3.0 M HCl is more concentrated. Molarity (M) measures the amount of solute in moles per liter of solution, thus a higher molarity indicates more solute particles in the same volume, leading to a higher concentration. Surface area, temperature, and opacity are not directly related to the molarity of a solution. Hence, the correct answer is C. A more concentrated hydrochloric acid solution will have a higher number of hydronium ions, indicating a stronger acidic property and a lower pH compared to less concentrated solutions. Nothing in the solute's concentration inherently affects surface area, temperature, or how opaque the solution is.