Correct option is D
The lower critical point is the temperature at which a change starts to occur when heating steel. Specifically, it is the temperature at which the steel begins to undergo a phase transformation, starting from a ferrite or pearlite structure to an austenitic structure. This temperature is crucial in heat treatment processes, such as annealing or hardening, where controlled heating and cooling are necessary to achieve the desired material properties. The lower critical point marks the beginning of this transformation during heating.
