To calculate the volume of a cuboid (the shape shown) you need 3 values:
Width, length and height.
As we already have the width and length, we need the height and we can determine the height from the picture of the shape that is shown. In this case the height would be either RU or ST. We look at the possible answers to see if any of the possible answers match our possible solutions - in this case ST.
Therefore the answer would be a) ST