What is a plane surface?A.A curved surface which extends in all directions is called a plane surface.B.A line segment which extends in all directions is called a plane surface.C.A cubic surface which extends in all directions is called a plane surface.D.A flat surface which extends in all directions is called a plane surface.

A plane surface is a flat surface that extends infinitely in all directions. It has no curvature or bends, and any straight line drawn on a plane surface will lie entirely on that surface.

Now, let's examine the options:

A. A curved surface which extends in all directions is called a plane surface.
This option is incorrect because a plane surface is flat, not curved.

B. A line segment which extends in all directions is called a plane surface.
This option is incorrect because a line segment is not a surface, and it does not extend in all directions. A plane surface is two-dimensional.

C. A cubic surface which extends in all directions is called a plane surface.
This option is incorrect because a cubic surface is 3D, while a plane surface is 2D.

D. A flat surface which extends in all directions is called a plane surface.
This option is correct. A plane surface is a flat, two-dimensional surface that extends infinitely in all directions.

So, the correct answer is:
D. A flat surface which extends in all directions is called a plane surface.