A line is an undefined term because it

Step 1: Understand what an undefined term is in geometry

In geometry, certain fundamental terms are called undefined terms. These are terms that do not have a formal definition but are understood through examples, descriptions, and intuitive concepts.

Step 2: Recognize the three main undefined terms

• Point
• Line
• Plane

These are considered undefined because they are the building blocks of geometric definitions and cannot be defined using simpler terms.

Step 3: Why is a line undefined?

• A line is infinitely long and has no thickness.
• It cannot be defined using other geometric figures, because any attempt to define it uses concepts that themselves depend on the idea of a line.

Step 4: Eliminate incorrect options

• Line segment: A defined part of a line with two endpoints.
• Vertex, focus, major axis: These are defined using other geometric terms.

Correct Explanation:

A line is an undefined term because it has no formal definition and must be understood through intuitive meaning rather than using previously defined terms.

Final Answer: line segment (as part of the reasoning, though the question is conceptual)