Final answer:
It is important to know the hardness of substances when sharpening a knife because hard substances resist abrasion, allowing the maintenance of a sharper knife's edge which increases the pressure and cutting ability of the knife.
Step-by-step explanation:
Knowing the hardness of certain substances is important when sharpening a knife because a hard substance resists abrasion and maintains the knife's edge. Option D is correct: Hard substances resist abrasion and maintain the knife's edge. By using a harder material, the sharpener creates a fine edge on the blade with reduced wear and tear. Harder materials are less prone to becoming deformed or worn away as the knife is dragged across them. The Mohs scale of mineral hardness can be used to compare the hardness of different materials for this purpose.
Pressure is an important concept to consider as it is related to the sharpness of a knife and its cutting ability. A sharper knife has a smaller surface area at its cutting edge, which means that the same force applied on a smaller area results in greater pressure, hence a greater ability to cut. Therefore, the hardness of the sharpening material ensures that the knife can be honed to a sharp edge that can effectively concentrate force and increase cutting pressure.