Question

which factor is most important to keep the same when comparing the hardness of solids?
a. the amount of each substance
b. the intensity of the light
c. the temperature of the hot plate
d. the method of making a scratch on the material

Explanation:

When comparing the hardness of solids, the method of making a scratch (as in hardness tests like Mohs scale which rely on scratching) must be consistent. Amount of substance (A) doesn't affect hardness (a property). Light intensity (B) is irrelevant to hardness. Hot - plate temperature (C) might affect some materials but is not the most fundamental factor for comparing hardness as a general property. The method of scratching (D) ensures a standardized way to measure the resistance to deformation (hardness).

Answer:

D. The method of making a scratch on the material