Final answer:
The density of the marble dropped into the graduated cylinder is calculated by dividing its mass (4 grams) by the volume of water displaced (8 ml), resulting in a density of 0.5 g/ml.
Step-by-step explanation:
The task is to determine the density of a marble by using the water displacement method in a graduated cylinder. When the marble, with a mass of 4 grams, is dropped into the cylinder containing water, the water level rises from 20 ml to 28 ml. The volume of the marble is thus 28 ml - 20 ml = 8 ml. The density is mass divided by volume, so the density of the marble is 4 g / 8 ml = 0.5 g/ml. Therefore, the correct answer is A. 0.5 g/ml.