Final answer:
The volume of the solid is b) 72 cm³.
Step-by-step explanation:
To find the volume of the solid, we can compare the change in water level before and after the solid is immersed. Initially, the water level is at 7cm, and after immersing the solid, it rises to 9cm. The difference in the water levels, 9cm - 7cm = 2cm, represents the volume of the solid.
Since the base of the box is a square with a side length of 6cm, the volume of the solid can be calculated as 6cm * 6cm * 2cm = 72 cubic centimeters.
Therefore, the correct answer is (b) 72 cm³.