Malleable cast iron is produced:
A.By adding magnesium to molten cast iron
B.From white cast iron by annealing process
C.By quick cooling of molten cast iron
D.None of these

Hint:Cast iron is a combined group of alloys of iron and carbon having a carbon content of more than 2% and less than 4% in addition to varying amounts of manganese, silicon and traces of phosphorus and sulphur.

Complete answer:
Cast iron is produced by various heat treatments and processing techniques. As a result, different types of cast iron are produced namely, grey cast iron, white cast iron, ductile cast iron, graphite cast iron and malleable cast iron.
Malleable cast iron is a modified form of white cast iron which is modified by heat treatment. It is formed by heating white cast iron at high temperatures and then cooled very slowly. The graphite separates out very slowly in this process, and thus the surface tension has time to form it into spheroidal particles instead of flakes. They have blunt boundaries, which decreases the stress concentration problems faced in other types of cast iron. In general, we can say that the properties of malleable cast iron resemble those of steel.

Thus, the correct answer is B.
Note:
The white cast iron is converted into malleable cast iron following a two stage heat treatment process till a condition is reached so that most of the carbon content of the cast iron is in the form of irregularly shaped nodules of graphite, which is known as temper carbon. Malleable cast iron has a structure that consists of temper carbon, pearlite and ferrite. It has properties in its microstructure that make it ideal for use where toughness and malleability are required.