**The Carbon Content of Structural Steel**

The correct answer to the question is option 'B', which states that the carbon content of structural steel ranges from 0.10 to 0.25%. In order to understand why this is the correct answer, let's delve into the properties and composition of structural steel.

**Structural Steel Composition:**

Structural steel is a type of steel that is widely used in construction and engineering applications due to its high strength, durability, and versatility. It is predominantly composed of iron, with small amounts of carbon and other elements. The carbon content is an important factor in determining the properties of the structural steel.

**Effect of Carbon Content on Steel:**

Carbon is one of the main alloying elements in steel and has a significant impact on its properties. The carbon content affects the hardness, strength, and ductility of the steel. Higher carbon content generally results in increased hardness and strength, but decreased ductility. On the other hand, lower carbon content leads to improved ductility but reduced strength.

**Carbon Content Range in Structural Steel:**

Structural steel is designed to have a balance between strength and ductility, making it suitable for various construction applications. The carbon content in structural steel is typically controlled within a specific range to achieve the desired properties. The correct answer states that the carbon content of structural steel ranges from 0.10 to 0.25%. Let's understand why this range is commonly used:

1. **Less than 0.1% Carbon (Option A):**
- Carbon content less than 0.1% is generally considered low carbon steel.
- This low carbon content provides excellent ductility and weldability.
- However, it may result in reduced strength and hardness, making it less suitable for structural applications where higher strength is required.

2. **0.10 to 0.25% Carbon (Option B):**
- This range is commonly used in structural steel.
- It provides a good balance between strength and ductility.
- The moderate carbon content enhances the strength and hardness of the steel while maintaining reasonable ductility.
- This range is suitable for various structural applications where both strength and ductility are essential.

3. **0.25 to 0.60% Carbon (Option C):**
- This range is considered medium carbon steel.
- Medium carbon steel offers higher strength and hardness compared to low carbon steel.
- However, it may have reduced ductility and weldability.
- This range is typically used in applications where higher strength is required but not at the expense of excessive hardness or brittleness.

4. **0.60 to 1.00% Carbon (Option D):**
- This range is considered high carbon steel.
- High carbon steel offers excellent strength and hardness.
- However, it has reduced ductility and weldability.
- High carbon steel is generally used in specialized applications such as cutting tools, springs, and high-strength wires.

In conclusion, the carbon content of structural steel typically ranges from 0.10 to 0.25%. This range provides a good balance between strength and ductility, making it suitable for various construction applications.