The correct answer is C) Round nose. Round nose chisels are specifically used for cutting oil grooves. The rounded shape of the chisel tip allows it to easily carve out the curved profile of an oil groove, which is necessary for proper lubrication. The other options would not be suitable for this task - square nose and flat cold chisels have straight, flat edges that cannot effectively create the desired curved oil groove shape, while a diamond point chisel is typically used for more precise, detailed work rather than cutting larger oil grooves.