ANS:B - hypo-eutectoid steel.

An alloy of Fe with 0.4% carbon (C) is classified as hypo-eutectoid steel. In the iron-carbon phase diagram, the eutectoid composition, where the steel transforms completely into pearlite during cooling, is around 0.77% carbon. Therefore, an alloy with a carbon content less than this (such as 0.4% carbon) is considered hypo-eutectoid. Hypo-eutectoid steels undergo eutectoid transformation during cooling, where the steel transforms partially into pearlite and ferrite. In the case of hypo-eutectoid steels, there is not enough carbon to form pearlite exclusively. Instead, the excess carbon is present as free cementite or as a mixture of cementite and pearlite. Therefore, the correct classification for an alloy of Fe with 0.4% carbon is hypo-eutectoid steel.