Which substance is represented by the prefix "hydro-"?

The prefix "hydro-" is derived from the Greek word "hydor," meaning water. In scientific terminology, it often signifies a relation to water or the presence of water. In this context, "hydro-" specifically indicates substances or compounds that contain water or are associated with it.

While the other options present different elements or compounds, none encapsulate the water-related meaning inherent in the prefix "hydro-." For example, oxygen is a component of water but does not directly represent water itself. Carbon is a fundamental element found in many organic compounds, and hydrogen, while crucial as a component of water (H2O), is not equivalent to water on its own. Thus, "hydro-" aptly refers to water in this context.