To solve this, you subtract the initial volume from the final volume. So, 29.5 mL minus 25.0 mL is 4.5 mL. Technically, at this point you need to say 4.50 mL, because of sig figs. The volume of the stone will not be in mL, because the stone is not a liquid or a gas, so you must convert it to cm3. So, your answer is (a), 4.50 cm3.
Hope this helps!
