**Answer:**

The correct answer is option A, "plane".

A **plane** is a flat surface that extends infinitely in all directions. It is a two-dimensional surface that has length and width but no thickness. A plane can be thought of as an imaginary sheet of paper that goes on forever in all directions.

Here are some key points to understand about planes:

**1. Definition:** A plane is a flat, two-dimensional surface that extends infinitely in all directions.

**2. Characteristics of a Plane:**
- A plane has no thickness.
- It is infinitely large and extends indefinitely in all directions.
- It is a flat surface that can be represented mathematically.
- A plane has two dimensions - length and width - but no height.

**3. Representation:** A plane can be represented by drawing a flat surface on a piece of paper or by using mathematical equations to describe its properties.

**4. Examples:** Some examples of planes include:
- The surface of a table or a whiteboard.
- The floor or ceiling of a room.
- The surface of a lake or a pool.
- The surface of a piece of paper.

**5. Use in Geometry:** Planes are an important concept in geometry. They are used to define and describe geometric shapes and figures. For example, a square is a flat shape that lies in a plane, and a cube is a three-dimensional shape made up of six square faces that are all in different planes.

In conclusion, a flat surface that extends indefinitely in all directions is called a plane. It is a two-dimensional surface with length and width but no thickness. Planes are important in mathematics and geometry for defining and describing shapes and figures.

1. A plane is a flat surface that extends infinitely in all directions
2. It can be thought of as an imaginary sheet of paper that goes on forever in all directions
3. It is used in geometry