What does this mean: a saturated solution may be quite dilute because some substances have a very low solubility. i don't understand what this is saying.

Here's how I would explain it.

Explanation:

Think of it this way.

When you mix solutions of silver nitrate and sodium chloride, you get an immediate precipitate of silver chloride. The equation is

Ag⁺(aq) + Cl⁻(aq) ⟶ AgCl(s)

Now, take some AgCl and stir it vigorously with water.

You won't see much happening, because the AgCl is has such a low solubility. Not much of it will go into solution. And yet, a small amount of it does dissolve until the solution is saturated.

The concentration of AgCl in the saturated solution is

about 0.000 01 mol·L⁻¹.

I hope you will agree that this is a dilute solution even though it is saturated with AgCl.

c is the answer because when something is left after something evaporates that shows the chemical change.