To measure the volume of an object using the water displacement method, you submerge the object in a graduated cylinder filled with water. The volume of water displaced by the object is equal to the volume of the object itself. In this case, the girls need to read the new water level in the graduated cylinder after submerging the rubber band. The difference between the initial water level and the new water level gives the volume of the rubber band. Therefore, the appropriate word to fill in the blank is 'Reading', as it refers to the action of observing the measurement on the graduated cylinder.