The boundaries of solids are surfaces.

Explanation:
Solids are three-dimensional objects that have length, width, and height. The boundaries or outer surfaces of solids are what define their shape and determine their volume. These surfaces enclose the solid and separate it from its surroundings.

Here are some key points to understand why the correct answer is option 'B' - surfaces:

1. Definition of a solid:
- A solid is a three-dimensional object that has a definite shape and volume.
- It is composed of tiny particles closely packed together.

2. Boundaries of solids:
- The boundaries or surfaces of solids are what separate them from their surroundings.
- These surfaces enclose the solid and define its shape.
- The boundaries can be smooth, rough, curved, or flat depending on the solid.

3. Examples of solid boundaries:
- Consider a cube, which is a solid with six square faces.
- The boundaries of the cube are the six square surfaces that enclose it.
- Each surface is a boundary that separates the cube from its surroundings.
- Similarly, other solids like a sphere, cylinder, pyramid, etc., have surfaces that enclose them.

4. Characteristics of solid boundaries:
- Solid boundaries are continuous and have no breaks or holes.
- They are three-dimensional and can be curved, flat, or a combination of both.
- The boundaries can have different properties like smoothness, roughness, reflectivity, etc., depending on the solid and its composition.

5. Importance of solid boundaries:
- The boundaries of solids play a crucial role in determining their properties and behavior.
- They define the shape, volume, surface area, and other physical characteristics of the solid.
- The boundaries also influence how solids interact with their surroundings, such as heat transfer, chemical reactions, etc.

In conclusion, the correct answer is option 'B' - surfaces. The boundaries of solids are surfaces that enclose and define their shape. These surfaces separate the solid from its surroundings and play a significant role in determining its properties.