Correct option is B
Martensite is the hardest constituent of steel. It is formed by the rapid cooling (quenching) of austenite, a process that traps carbon atoms within the iron lattice, resulting in a highly strained and distorted structure. This gives martensite its characteristic high hardness and brittleness.
Bainite is a phase that is less hard than martensite but tougher.
Austenite is a softer phase that is stable at high temperatures.
Ledeburite is a mixture of cementite and austenite found in very high carbon steels, but it is not as hard as martensite.
Thus, the hardest constituent of steel is martensite.
