Letter C is the answer to this or Perchloric Acid. This is a type of mineral acid that is usually found in aqueous solutions. This is colorless so it will be hard to distinguish this acid from other similar looking acids. What makes this different from the others is that it is stronger and more acidic.
If it would be compared to sulfuric acid, it will definitely be stronger. This is actually considered to be the strongest out of all the known acids for this type of element. In order to figure out how strong the acid is, you may want to check how much oxygen the acid contains. The more oxygen, the stronger the acid is going to be.