Ductile cast iron, also known as nodular cast iron or spheroidal graphite cast iron, is a type of cast iron that contains carbon in the form of spherical nodules. It is a highly versatile material that combines the strength and durability of cast iron with the ductility and toughness of steel. The correct answer to the question is option 'D', which states that all of the mentioned statements are correct.

Below is a detailed explanation of each statement:

a) Also called nodular cast iron:
Ductile cast iron is commonly referred to as nodular cast iron because of its unique microstructure. Unlike traditional cast iron, which consists of flake graphite, ductile cast iron contains graphite nodules or spheroids. These nodules improve the mechanical properties of the material, making it more ductile and less brittle.

b) Also called spheroidal graphite cast iron:
Ductile cast iron is also known as spheroidal graphite cast iron due to the presence of graphite nodules in a spherical shape. These nodules provide excellent resistance to crack propagation, which enhances the material's toughness and impact resistance.

c) Carbon is present in the form of spherical nodules:
One of the defining characteristics of ductile cast iron is the presence of graphite nodules. These nodules are formed during the casting process by adding small amounts of magnesium or cerium to the molten iron. These elements promote the formation of graphite in a nodular or spherical shape, rather than the flake-like structure found in traditional cast iron.

The presence of graphite nodules gives ductile cast iron its unique properties, such as high tensile strength, excellent machinability, and good wear resistance. The nodules also act as stress concentrators, providing a mechanism for energy absorption and reducing the risk of catastrophic failure.

In summary, ductile cast iron is a type of cast iron that contains carbon in the form of spherical nodules. It is also known as nodular cast iron or spheroidal graphite cast iron. The presence of these nodules enhances the material's ductility, toughness, and overall mechanical properties. Therefore, option 'D' is the correct answer as all the mentioned statements are true.