The correct answer is: C. 13 kg/mm2
Whitworth compressed steel is obtained when molten steel is subjected to a pressure of 13 kg/mm2. This process is known as cold working, and it results in a stronger and more durable steel.
Option A: 5 kg/mm2 is too low of a pressure to produce Whitworth compressed steel.
Option B: 9 kg/mm2 is a higher pressure than is necessary to produce Whitworth compressed steel, but it is still within the range of pressures that can be used to cold work steel.
Option C: 13 kg/mm2 is the correct pressure to produce Whitworth compressed steel.
Option D: 15 kg/mm2 is too high of a pressure to produce Whitworth compressed steel. It is possible that this pressure could cause the steel to break or become brittle.