Answer:
A. A term that is a building block for other definitions in geometry.
Explanation:
In Geometry, undefined terms refer to concepts like point, line and plane. Which are concepts that cannot be defined, they are too intuitive. Undefined terms are always true, they cannot be prove because those concepts are too primitive.
It's important to know that Geometry develops another complex defintions from these undefined terms, beacuse point, line and plane are the fundamentals of Geometry.
Therefore, the right answer is A.
Other choices are just wrong, because undefined terms aren't condition statments, they cannot be proven and they aren't oppsite of an original statement.