The correct answer is option D) Ductility.

Explanation:
- Carbon is one of the main alloying elements in steel and it has a significant impact on its properties.
- Ductility is the ability of a material to deform under tensile stress without breaking or fracturing.
- Increasing the percentage of carbon in steel has a detrimental effect on its ductility.
- This is because higher levels of carbon cause the steel to become more brittle.
- Brittleness is the opposite of ductility, where a material is prone to fracture without significant deformation.
- The brittleness of steel increases with an increase in carbon content.
- This means that as the carbon percentage in steel increases, its ductility decreases, making it more prone to fracture.
- On the other hand, strength refers to the ability of a material to withstand applied stress without deformation or failure.
- The strength of steel can be improved by increasing the carbon content, as carbon contributes to the formation of stronger and harder phases.
- Similarly, hardness is the resistance of a material to indentation or scratching.
- Increasing the carbon content in steel generally increases its hardness.
- However, both strength and hardness are separate properties from ductility.
- While increasing the carbon content in steel can enhance its strength and hardness, it comes at the expense of ductility.
- Therefore, the correct answer is option D) Ductility, as increasing the percentage of carbon in steel decreases its ductility.