Discussion
At room temperature, plain carbon steel contains:
Answer: C
The keyed answer is pearlite and cementite, and the question is loosely worded, so read the answer as the key's.
Plain carbon steel runs from about 0.02% to 2% carbon. Above the eutectoid composition of 0.8% carbon (hypereutectoid steel) the structure at room temperature is pearlite with free cementite along the grain boundaries, which is the keyed option. Pearlite is itself a fine mixture of ferrite and cementite.
The other options each fit only some steels. Ferrite and pearlite is the structure of the more common low and medium carbon steels below 0.8% carbon, so many candidates will choose it, and it is the option to defend if the question meant a typical structural steel. Ferrite and cementite are the two phases present in every plain carbon steel at equilibrium, but the option does not name pearlite. Ferrite, pearlite and cementite all together do not occur in one equilibrium steel, because free ferrite and free cementite are never present beside pearlite in the same steel.
The official key gave C, and it is kept here; the stem does not say which carbon range it means.
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