The hardness of steel depends upon the

Correct Answer: B: Amount of carbon it contains

Here's why:

* Hardness Explained: Hardness is a material's resistance to permanent deformation, like scratching or indentation.

* Role of Carbon in Steel: Steel is primarily iron (Fe) with added carbon (C). The amount of carbon significantly impacts its hardness.

* Carbon and Hardness Relationship: Generally, increasing the carbon content in steel increases its hardness and strength. Carbon atoms interfere with the movement of iron atoms, making it more difficult to deform the steel.

* Why other options are less accurate:
* A: Amount of cementite it contains: Cementite (Fe3C) is a compound of iron and carbon. While it contributes to hardness, the *amount of carbon* present is the more direct factor.
* C: Contents of alloying elements: Alloying elements (like chromium, nickel, etc.) can definitely affect hardness, but the *primary* influence is from carbon.
* D: Method of manufacture of steel: Manufacturing methods (like heat treatment) can alter the microstructure and hardness of steel, but they don't fundamentally change its inherent hardness potential, which is set by the carbon content.