Because solid has definite shape so that's why solid has a plane surface

The correct answer is option 'C' - surfaces.

Explanation:
A solid is a three-dimensional object that has length, width, and height. It occupies space and has boundaries that define its shape. These boundaries are known as surfaces. Surfaces are two-dimensional and can vary in shape and size.

Let's understand this further by breaking down the answer into headings and key points:

1. Definition of solids:
- Solids are three-dimensional objects that have length, width, and height.
- They occupy space and have boundaries that define their shape.

2. Boundaries of solids:
- Boundaries of solids are the surfaces that enclose them.
- These surfaces can be flat, curved, or a combination of both.
- The shape of the surfaces depends on the shape of the solid.

3. Curves as boundaries:
- Curves can be part of the boundaries of some solids.
- For example, a cylinder has curved surfaces that define its shape.
- The boundaries of a sphere are also curved surfaces.

4. Lines as boundaries:
- Lines cannot be the boundaries of solids.
- Lines are one-dimensional and do not enclose space.
- While lines can be present within solids, they do not define the overall shape of the solid.

5. Surfaces as boundaries:
- Surfaces are two-dimensional and can enclose space.
- They are the boundaries that define the shape of a solid.
- Surfaces can be flat, curved, or a combination of both.

6. Points as boundaries:
- Points cannot be the boundaries of solids.
- Points are zero-dimensional and do not enclose space.
- While points can exist within solids, they do not define the overall shape of the solid.

In conclusion, the boundaries of solids are surfaces. Surfaces are two-dimensional and can be flat or curved. They enclose space and define the shape of a solid. Curves, lines, and points cannot be the boundaries of solids as they are one-dimensional or zero-dimensional.