the floor the foundation the plumbing the walls
I gave my students this multiple choice question: During the construction of a new building, workers dug round holes in the ground and filled the holes with steel and concrete. Which of the following parts of the building was being built during this stage of construction?

Only one answer is correct among these 4 choices:
- the floor
- the foundation
- the plumbing
- the walls

Could you tell me which one is correct?

the foundation