When an object is submerged in water, it displaces its own volume of water. The water level in the cylinder will rise by the volume of the object. The final measurement is 45 mL.
The object will displace its own volume of water when it is submerged. This means that the water level in the cylinder will rise by 15.0 cm³. The new water level will be at the 45 mL mark on the graduated cylinder.
The object could be anything that has a volume of 15.0 cm³ and a mass of 21.0 g. It could be a small rock, a piece of metal, or even a chunk of bread.
The final measurement is 45 mL. This is written with the correct number of significant figures, because the graduated cylinder is only marked to the nearest mL.