Correct Answer: base

Solution : The correct choice is the third option .

Foundation is a body or ground upon which something is built up or overlaid. Base is the bottom part of an object, on which it rests, making it the correct synonym.

The meanings of the other options are as follows:

• The top is the highest or uppermost point.
• A building is an enclosed construction over a plot of land
• Structure is the construction of a building; how it is built.