Which of the following best describes hydrocarbons?

The description of hydrocarbons as compounds that contain only hydrogen and carbon is accurate because hydrocarbons are organic molecules characterized by the presence of these two elements. They form the backbone of many organic compounds and are fundamental in biological systems and industrial applications. Hydrocarbons can be classified into various categories, including aliphatic and aromatic hydrocarbons, depending on their structure and types of bonds present. This structural composition is essential for understanding their behavior, reactivity, and role in biochemical processes.

Other choices reflect different attributes that do not specifically characterize hydrocarbons. For instance, while some hydrocarbons may be present in living organisms, they are not universally present in all, thus ruling out the broader claim made in one of the options. Additionally, the solubility of hydrocarbons in water is generally low due to their nonpolar nature, and while certain hydrocarbons can be reactive, they are not universally classified as highly reactive in biological systems. Therefore, the clear definition provided by the correct choice highlights the unique composition of hydrocarbons, which is the defining characteristic for this class of compounds.