What characteristic defines a corrosive substance?

A corrosive substance is characterized by its ability to eat away at or damage materials, which is precisely why option C is the correct choice. Corrosive substances can react chemically with metals, skin, and other materials, leading to degradation or destruction. This can occur through various mechanisms, including acid-base reactions, oxidation, or hydrolysis, depending on the specific properties of the substance.

On the other hand, a corrosive substance does not inherently promote growth (as mentioned in option A); indeed, its destructive properties typically inhibit growth rather than support it. While many corrosive substances can dissolve in water, not all of them do so easily, which makes option B less suitable. Lastly, enhancing the effectiveness of pesticides (option D) is not a defining property of corrosives; some corrosive agents can be harmful to both target organisms and non-target species, including plants and animals, making them inappropriate for enhancing pesticide effectiveness. Thus, the key characteristic that makes a substance corrosive is its ability to eat away or damage materials, making option C the correct choice.