What physical property of the rock can be directly determined when the rock is placed in the overflow container? The diagram below shows a rock suspended above an overflow container filled with water up to the overflow spout. A graduated cylinder is positioned next to the container to collect water that comes out of the overflow spout.
It might be tempting to assume that mass can be determined by sinking a rock into a measuring cylinder of liquid, but only the volume can actually be determined that way. Simply, the volume displaced by the object pushes the level of liquid up so that the before and after measurement will give the volume of the object accurately. So the answer to the question is volume.