Boundaries of solids are

Options

A. surface
B. curves
C. lines
D. points

Explanation

To solve the question "Boundaries of solids are," we need to analyze the properties of solids and their boundaries. Let's break it down step by step.

### Step-by-Step Solution:

1. **Understanding Solids**:
- Solids are three-dimensional figures. They occupy space and have length, breadth, and height.

**Hint**: Remember that three-dimensional figures have volume and can be visualized in space.

2. **Identifying a Solid**:
- Let's take a cuboid as an example of a solid. A cuboid is defined by its three dimensions: length, breadth, and height.

**Hint**: Think of common objects like boxes or bricks that have a cuboid shape.

3. **Analyzing the Faces of the Cuboid**:
- A cuboid has six faces (or surfaces). Each face is a two-dimensional shape. For instance, the top face of the cuboid has a length and a breadth.

**Hint**: Visualize the cuboid and count its faces. Each face is flat and can be measured in two dimensions.

4. **Understanding Boundaries**:
- The boundaries of a solid refer to its surfaces. Since each face of a solid is a two-dimensional figure, we can conclude that the boundaries of solids are surfaces.

**Hint**: Consider how you would describe the outer layer of any solid object; it is made up of surfaces.

5. **Conclusion**:
- Therefore, the boundaries of solids are best described as surfaces.

**Hint**: Think about how surfaces enclose the volume of a solid.

### Final Answer:
The boundaries of solids are surfaces.