Option B: Strength of chemical bonds
Hardness defines the strength, it can not be compared with surface roughness as a smooth surface can be harder or difficult to break than a rough surface. The mass and weight of a substance matters while comparing density of substance or how heavy it is but it is not directly proportional to the hardness of the substance. Thus, equal masses of two substance can not be considered equally hard. Hardness depends on the inter-molecular interaction or forces within a substance, if the inter-molecular interactions are strong or the strength of chemical bonds is strong then the substance is more hard.
Therefore, hardness is directly proportional to strength of the chemical bonds.
I only knew a few of these:
1. Curium
2. Copper
8. Iron
11. Barium
13. Zinc
14. Europium
29. Au (gold)
