Correct option is (c) 1.6 mm
To elaborate: The given sample of material is heated above the upper critical temperature. It is then quenched at one end at 25^oC. After cooling, a flat is ground along the length of the bar. Rockwell C hardness readings are then taken for every 1.6 mm along the length from the quenched end, which are then plotted in the form of a graph.