Option C is correct answer ......

manganese increases hardenability and tensile strength of steel, but to a lesser extent than carbon.
It is also able to decrease the critical cooling rate during hardening, thus increasing the steels hardenability much more efficient than any other alloying elements.

Manganese is used to make the blades of bulldozers, bucket wheel excavators and other earth moving equipment contain iron.

Explanation:

Alloy steel is a type of steel that contains various alloying elements in addition to iron and carbon. These alloying elements are added to enhance the mechanical properties of the steel, such as hardness, strength, and toughness.

Work Hardening:
Work hardening, also known as strain hardening, is a process in which the material becomes stronger and harder as it is deformed or worked. This is achieved by introducing dislocations in the crystal structure of the material, which hinder the movement of atoms and make it more resistant to deformation.

Blade of Bull Dozers and Bucket Wheel Excavators:
The blades of bull dozers and bucket wheel excavators are subjected to heavy loads and abrasive wear during their operation. Therefore, these blades need to be made from a material that is strong, tough, and wear-resistant. Alloy steel is an ideal choice for these applications due to its excellent combination of strength, hardness, and wear resistance.

Elements in Alloy Steel:
1. Iron: Iron is the main component of steel and provides its basic mechanical properties.
2. Carbon: Carbon is a key alloying element in steel that increases its hardness and strength. It forms carbides with iron, which contribute to the hardness of the steel.
3. Chromium: Chromium is commonly added to alloy steel to improve its corrosion resistance and wear resistance. However, it is not mentioned in the given options.
4. Silicon: Silicon is added to steel to improve its strength and resistance to high temperatures. It also helps in deoxidizing the steel during the manufacturing process.
5. Manganese: Manganese is commonly added to steel as a deoxidizer and to improve its strength, hardness, and toughness. It also forms manganese sulfides, which improve machinability.
6. Magnesium: Magnesium is not typically added to alloy steel. It is used in other applications, such as aluminum alloys.

Correct Answer:
The correct answer is option 'C' - Manganese. Manganese is commonly added to alloy steel to improve its strength, hardness, and toughness. It also acts as a deoxidizer during the steel manufacturing process.